President Trump appeared to back the claims of a lawyer for his presidential transition team that special counsel Robert S. Mueller III improperly obtained Trump associates’ emails, saying Sunday that “a lot of lawyers thought that was pretty sad.”

Trump also said he has no plan to fire Mueller, who is investigating Russian interference in the presidential election, including the possibility of coordination with the Trump campaign. Mueller’s investigation has resulted in charges against two people and guilty pleas by two others, including former national security adviser Michael Flynn.

“Not looking good. It’s not looking good,” Trump said when asked whether Mueller had received the transition documents improperly.

A lawyer for Trump’s transition team contends that Mueller should not have been able to obtain a trove of emails from the period between Trump’s 2016 election victory and his Jan. 20 inauguration without the consent of transition officials.

The batch of emails totaling thousands of pages of communications was provided to Mueller by the federal General Services Administration, the lawyer representing the organization known as Trump for America said in a letter delivered to congressional investigators Saturday.

“This morning we sent a letter to Congress concerning the unauthorized sharing of private and transition emails with the Mueller team,” lawyer Kory Langhofer said in an interview Saturday.

A spokesman for Mueller’s team rejected the allegations of impropriety.

“When we have obtained emails in the course of our ongoing criminal investigation, we have secured either the account owner’s consent or appropriate criminal process,” said Peter Carr, a spokesman for Mueller’s office.

The letter from Langhofer is the latest in a series of legal and public relations moves by Trump’s allies to attempt to undermine Mueller’s investigation and portray it as politically motivated.

“It’s quite sad to see that. My people were very upset,” Trump told reporters at the White House about the transition emails upon his return from a weekend trip to Camp David.

“I can’t imagine there’s anything on them, because as we said there’s no collusion,” Trump said. “There was no collusion whatsoever.”

Some of the president’s supporters have urged him to fire Mueller, but the White House has consistently said there is no discussion about getting rid of the special counsel.

Special Counsel Robert S. Mueller III departs after a meeting with members of the Senate Judiciary Committee on June 17. (J. Scott Applewhite/AP)

[Inside Trump’s legal team: Trying to protect the president from Mueller’s ‘killers’]

The letter from Langhofer, who was counsel to Trump for America, alleged that career employees of the GSA improperly provided privileged communications to investigators working for Mueller.

Transition documents are private property, not government records, the transition team contends. The letter invokes federal law and what Langhofer calls decades of precedent to argue that Mueller overstepped.

The transfer of transition documents is “unlawful conduct that undermines the Presidential Transition Act of 1963,” the letter said, “and will impair the ability of future presidential transition teams to candidly discuss policy and internal matters that benefit the country as a whole.”

The letter was sent to the House Oversight and Government Reform Committee as well as the Senate Homeland Security and Governmental Affairs Committee.

The Trump transition alleges that the handover was done by “career staff at the General Services Administration” and suggested that those employees may have had political motives.

But some legal experts challenged Langhofer’s charge that anything improper occurred.

Randall Eliason, a former federal prosecutor who teaches white collar crime at George Washington University Law School, said it was not at all surprising that Mueller’s team sought Trump transition emails. “It would be almost prosecutorial misconduct for them not to,” he said. He said it was also not surprising that Mueller would ask GSA for emails sent using government accounts.

“It’s not your personal email. If it ends in .gov, you don’t have any expectation of privacy,” he said.

But he said if Trump’s team had a valid legal claim, there is a standard avenue to pursue — they would file a sealed motion to the judge supervising the grand jury and ask the judge to rule the emails were improperly seized and provide a remedy, like requiring Mueller’s team to return the emails or excluding their use in the investigation.

“You go to the judge and complain,” he said. “You don’t issue a press release or go to Congress. It appears from the outside that this is part of a pattern of trying to undermine Mueller’s investigation.”

Eliason said he could think of “no apparent privilege” that would apply to emails sent between private citizens who have not yet joined the government, as Trump’s team suggested.

[Trump calls conduct at the FBI ‘disgraceful’ in latest criticism of bureau]

Trump’s lawyers were given a courtesy notice that the transition team lawyer planned to register a complaint, according to a person familiar with the discussion. The transition team lawyer learned of the records being provided to Mueller by GSA on Tuesday when a member of the transition was being interviewed by investigators and was shown a copy of their own email using transition account.

One White House adviser disputed pundits’ and lawyers’ claims that the only way the transition could object was to file a motion before the chief judge overseeing the grand jury.

“This wasn’t a subpoena pursuant to a grand jury. GSA just turned them over,” the adviser said.

The president’s lawyers are scheduled to meet with Muller’s team later this week for a status conference. His lawyers are expected to ask the special counsel if there are any other outstanding questions or materials that the team needs before it brings its probe to conclusion.

Democrats were quick to challenge the charge that either GSA or Mueller’s team acted improperly.

Rep. Elijah E. Cummings of Maryland, the senior Democrat on the Oversight and Government Reform Committee, said the 1963 Presidential Transition Act “simply does not support withholding transition team emails from criminal investigators.”

“The President’s lawyers have said they want to fully comply with Special Counsel Mueller’s investigation, so it is odd that they now suggest they would have withheld key documents from federal investigators,” Cummings said in a statement.

On Sunday, a Republican aide to the House Oversight and Government Reform Committee said the specific issues raised in Langhofer’s letter should be dealt with by the legal system — not Congress.

“To the extent the letter raises issues on how to improve subsequent transitions, the Committee takes the letter under advisement,” the aide said in an email.

[FBI director defends bureau’s integrity as GOP lawmakers press him on Trump, Clinton probes]

The GSA provided facilities to the Trump transition team in the weeks before Trump’s Jan. 20 inauguration. Langhofer claims that GSA had assured Trump for America that while it retained copies of transition records, it would not release them without consulting the organization.

GSA Deputy Counsel Lenny Loewentritt disputed Langhofer’s claim in an interview with BuzzFeed News Saturday. He told the news site that members of the transition team were informed that by using devices provided by GSA, materials “would not be held back in any law enforcement” requests.

GSA did not respond to a request for comment from The Washington Post.

Mueller’s team, Langhofer’s letter said, “has extensively used the materials in question, including portions that are susceptible to claims of privilege, and without notifying TFA or taking customary precautions to protect TFA’s rights and privileges.”

This story has been updated to include analysis of Langhofer’s letter from a legal expert, a response from a key Democratic congressman and additional context.

Mike DeBonis, Rosalind S. Helderman and Carol Leonnig contributed to this report.

Just before leaving his Defense Department job two months ago, intelligence officer Luis Elizondo quietly arranged to secure the release of three of the most unusual videos in the Pentagon’s secret vaults: raw footage from encounters between fighter jets and “anomalous aerial vehicles” — military jargon for UFOs.

The videos, all taken from cockpit cameras, show pilots struggling to lock their radars on oval-shaped vessels that, on screen, look vaguely like giant flying Tic Tacs. The strange aircraft — no claims are made about their possible origins or makeup — appear to hover briefly before sprinting away at speeds that elicit gasps and shouts from the pilots.

Elizondo, in an internal Pentagon memo requesting that the videos be cleared for public viewing, argued that the images could help educate pilots and improve aviation safety. But in interviews, he said his ultimate intention was to shed light on a little-known program Elizondo himself ran for seven years: a low-key Defense Department operation to collect and analyze reported UFO sightings.

The existence of the program, known as the Advanced Aviation Threat Identification Program, was confirmed officially for the first time Saturday by a Pentagon spokesman. The acknowledgment came in response to media inquiries, which were generated in part by a start-up company Elizondo has joined since retirement. The private company specializes in promoting UFO research for scientific and entertainment purposes.

Current and former Pentagon officials confirm that the Pentagon program has been in existence since 2007 and was formed for the purpose of collecting and analyzing a wide range of “anomalous aerospace threats” ranging from advanced aircraft fielded by traditional U.S. adversaries to commercial drones to possible alien encounters. It is a rare instance of ongoing government investigations into a UFO phenomenon that was the subject of multiple official inquiries in the 1950s and 1960s.

[Do we really want to know if we’re not alone in the universe?]

Spending for the program totaled at least $22 million, according to former Pentagon officials and documents seen by The Washington Post, but the funding officially ended in 2012. “It was determined that there were other, higher priority issues that merited funding and it was in the best interest of the DOD to make a change,” Pentagon spokesman Tom Crosson explained in a statement.

But officials familiar with the initiative say the collection effort continued as recently as last month. The program operated jointly out of the Pentagon and, at least for a time, an underground complex in Las Vegas managed by Bigelow Aerospace, a defense contractor that builds modules for space stations. It generated at least one report, a 490-page volume that describes alleged UFO sightings in the United States and numerous foreign countries over multiple decades.

Neither the Pentagon nor any of the program’s managers have claimed conclusive proof of extraterrestrial visitors, but Elizondo, citing accounts and data collected by his office over a decade, argues that the videos and other evidence failed to generate the kind of high-level attention he believes is warranted. As part of his decision to leave the Pentagon, he not only sought the release of videos but also penned a letter to Defense Secretary Jim Mattis complaining that a potential security threat was being ignored.

[No, we haven’t discovered alien megastructures around a distant star]

“Despite overwhelming evidence at both the classified and unclassified levels, certain individuals in the [Defense] Department remain staunchly opposed to further research on what could be a tactical threat to our pilots, sailors and soldiers, and perhaps even an existential threat to our national security,” Elizondo said in the letter, a copy of which was provided to The Post.

The first public revelations of the program came in a video conference aired in October by To the Stars Academy of Arts and Sciences, the firm Elizondo joined as a consultant after retiring from his Pentagon job. The New York Times and Politico reported the existence of the program on their websites Saturday. The Washington Post conducted several confidential interviews over two months with Elizondo and Christopher Mellon, a former deputy assistant secretary of defense for intelligence who also is an officer of the private firm.

Documents provided by the former officials included letters of support by former Senate majority leader Harry M. Reid (D-Nev.), a key backer of the initiative who helped secure funding for the program and sought to ensure a high degree of secrecy. Elizondo said knowledge of the program was limited, even within the Pentagon itself. He said the program had multiple enemies at senior levels of the department, from officials who were either skeptical or ideologically opposed to AATIP’s mission.

“I was honored to serve at the DOD and took my mission of exploring unexplained aerial phenomena quite seriously,” Elizondo said. “In the end, however, I couldn’t carry out that mission, because the department — which was understandably overstretched — couldn’t give it the resources that the mounting evidence deserved.”

It is difficult to draw conclusions about the nature of the unidentified vessels from the videos alone. Experts generally urged caution, explaining that reported UFO sightings often turn out of have innocuous explanations.

A retired Navy pilot contacted by The Post who was involved in a 2004 encounter depicted in one of the videos confirmed that the images accurately reflected his recollection of the events. The pilot would only speak on the condition of anonymity.

Elizondo, a 22-year veteran of the department who has held top security clearance and worked on secret counterintelligence missions, said he chose to join the private venture because he believed it was the best way to continue the work he was unable to complete as a government employee.

“I left to find an environment where investigating these phenomena is priority number one,” he said.