AUSTIN, Texas — Police said Monday that they believe three packages that exploded at homes and killed two people here are connected, raising fears that a bomber is on the loose in a city hosting tens of thousands of people for a world-renowned music and technology festival.

Authorities said it was too early to say what motivated the attacks and did not rule out the possibility of a hate crime. The two people killed in the explosions — a teenage boy and a 39-year-old man — were black, and a 75-year-old Hispanic woman was seriously injured.

The first explosion occurred March 2, when a package on the front porch of a northeast Austin home exploded and killed the man. At the time, police said his death was “suspicious” but believed it was an isolated incident with no continuing threat to the community.

That changed Monday morning, when a pair of packages detonated at homes several miles apart over a matter of hours. Investigators were still responding to the first — which killed a 17-year-old boy and seriously injured an adult woman — when the second blast detonated at a house farther south, seriously injuring the Hispanic woman. Police confirmed soon after that those cases were connected to each other and to the March 2 death.

Police and the FBI said they were working to solve the mystery and urged residents to be cautious in approaching packages left at their doorsteps unexpectedly. Officials said the packages that exploded did not come through the mail or a standard delivery service.

Austin Police Chief Brian Manley described the explosives as arriving in “box-type deliveries” but did not elaborate, citing the ongoing investigation. He said police did not know whether the victims who were killed or injured were the specific targets of the packages.

Austin is in the midst of hosting South by Southwest, a festival for which it has become famous, though authorities said that they did not believe the explosions were tied to that event.

The two victims killed in the explosions are relatives of prominent members of Austin’s African American community. The first, 39-year-old Anthony Stephan House, was the stepson of Freddie Dixon, a former pastor at a historic black church in Austin.

“This is a real mystery, and how all of this mystery comes together, I have no idea,” said Dixon.

He said he did not know of anyone who had a grudge against his stepson, who worked in construction, was married and had an 8-year-old daughter. But Dixon said he himself is good friends with Norman Mason, the grandfather of the teenager who was killed Monday. The teen has not been formally identified.

Mason is a dentist in East Austin who has for decades mentored African American student-athletes at the University of Texas at Austin. His wife, LaVonne Mason, is co-founder of the Austin Area Urban League.

LaVonne Mason confirmed her grandson was the 17-year-old victim who was killed in the explosion Monday morning. She declined to say anything further, citing an ongoing investigation.

“The investigation is going to take two to three days,” Mason said. “We are not at liberty to talk or discuss anything.”

Dixon said he wondered whether if the families’ connection might have motivated the crimes.

“Are you trying to say something to prominent African American families?” Dixon asked. “I don’t know who they’ve been targeting, but for sure, they went and got one of my best friends’ grandson. Somebody knew the connection.”

But Dixon noted that he did not know the woman injured in the third blast, whom relatives identified as Esperanza Herrera. They said her mother, Maria Moreno, suffered minor injuries.

Manley said that just as in the other bombings, the woman who was injured came outside her home, found a package and picked it up. That’s when it detonated.

“It’s not time to panic, but it’s time to be vigilant,” Manley said.

Later on Monday evening, according to the Austin American-Statesman, police temporarily shut down an area near the city’s downtown convention center because a guitar case in a trash can was deemed a suspicious package. The newspaper also said police received 63 suspicious-package calls through Monday afternoon, compared with two last Monday, as residents were being more cautious.

In the neighborhoods where bombs went off, residents were taking heed.

Lois Williams, 85, said the blast early Monday morning woke her up in her home about a block away.

“I just heard this – BOOM. It sounded like they were slamming the trash can lids,” Williams said.

Speaking with a reporter hours later in her driveway, Williams said she was not afraid because she doesn’t typically get packages but added that “I’m going to be looking.” A postal worker delivering Williams’ mail hugged her, saying, “Be careful.”

Rianne Philips, who lives next door to House, said her husband was the first to find House after the fatal blast.

She said that she was alarmed to hear about the latest bombings but also relieved that the police were now more focused on House’s death.

“They’re not going to let this slide,” Philips said. “It’s really sad, but this means there’s a lot of attention on this now.”

Isaiah Guerrero, 15, said he was spending the first morning of his spring break making music on his computer when he heard the third explosion go off just before noon Monday.

“It sounded like two cars hit each other, you know? Like, rammed each other,” Guerrero said.

The house shook, and so did his body, the teenager said. Guerrero then climbed up a tree and on top of his house. Within minutes, police and fire officials swarmed the scene, closing off streets. Guerrero, who lives behind the house where the bomb went off, said he couldn’t see the damage to the front of the house.

He echoed law enforcement officials in warning the public to pay attention to things like packages, “especially if you didn’t order something,” he said. Guerrero added: “I expected my spring break to be peaceful, not harmful.”

The Bureau of Alcohol, Tobacco, Firearms and Explosives said Monday it was dispatching members of its National Response Team (NRT) to help respond to the explosions. According to the agency, this group activates for “significant fire and explosion incidents,” considered those that are either large in scale or particularly complicated due to the size or scope.

In the past, that has included responding to the West, Tex., plant fire in 2013; a string of church fires in Texas; and the bombings in Oklahoma City and at the 1996 Olympic Games in Atlanta. The NRT works with other investigators to reconstruct scenes and determine what caused the fires or explosions; in cases involving bombings, the team also searches for evidence to be used in any prosecution that may follow.

Texas Gov. Greg Abbott (R) said his office is offering a $15,000 reward for information leading to the arrest of the person or people responsible for the “atrocious attacks.”

“I want to assure all Texans, and especially those in Austin, that local, state and federal law enforcement officials are working diligently to find those responsible for these heinous crimes,” Abbott said in a statement.

Manley said local and federal law enforcement agencies would ensure “every stop would be pulled out” to solve the cases.

“We are not going to tolerate this in Austin,” he said.

Berman, Wang, Zapotosky and Keith McMillan reported from Washington. Shane Harris in Austin contributed to this report, which has been updated.