Mr. Ullah acted alone, Mayor Bill de Blasio said, adding that no other devices had been found.

“Our lives revolve around the subway,” the mayor said. “The choice of New York is always for a reason, because we are a beacon to the world. And we actually show that a society of many faiths and many backgrounds can work.”

“The terrorists want to undermine that,” the mayor added. “They yearn to attack New York City.”

Mr. de Blasio spoke within hours of the attack. But the investigation by the Joint Terrorism Task Force was still in its preliminary stages.

Christina Bethea was in the underground walkway, headed to her job as a security guard, when the explosion nearly knocked her over, sending a haze of smoke into the corridor packed with commuters. She did not see where it came from, she said. “As soon as we heard ‘boom!’ we began to run,” she said. An hour after the attack, she stood outside the Port Authority, calling her mother and father in North Carolina to tell them she was O.K. “I feel good,” Ms. Bethea said. “I am alive!”

The authorities were searching Mr. Ullah’s residence on Ocean Parkway, pursuant to a federal warrant, one law enforcement official said. While no formal announcement had been made, both federal and local law enforcement officials indicated that Mr. Ullah would be prosecuted in federal court in Manhattan by the office of the acting United States attorney for the Southern District of New York, Joon H. Kim. The attack is being investigated by the Joint Terrorism Task Force, which is made up largely of F.B.I. agents and New York detectives, along with investigators from a score of other federal, state and local law enforcement agencies.

The attack roiled commutes across the region. All subway lines were directed to skip 42nd Street stops, according to the Metropolitan Transportation Authority. By late morning, only the A, C, and E were still skipping the stop. The Port Authority was evacuated for several hours; it reopened around 10:30 a.m. All morning, thwarted travelers spilled into the streets of Times Square, towing suitcases in bewildered silence. They gathered at police cordons stretched across 42nd Street, filming a scene of organized chaos as scores of emergency vehicles arrived at the scene every few minutes.

John Frank, 54, was standing on 42nd Street by the Port Authority exit when he felt tremors through the pavement. “That’s how strong it was,” he said. Everyone began to run. He stood on Eighth Avenue a few blocks away on Monday morning, shaken, leaning on a garbage pail for support. “In New York City, we are vulnerable to a lot of things,” he said. “These incidents are happening too frequently.”

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