NEW YORK — A would-be suicide attacker detonated a pipe bomb strapped to his body in the heart of Manhattan’s busiest subway corridor Monday, sending thousands of terrified commuters fleeing the smoke-choked passageways, and bringing the heart of Midtown to a standstill as hundreds of police officers converged on Times Square and the surrounding streets.

But the makeshift weapon failed to fully detonate, and the attacker himself was the only one seriously injured in the blast, which unfolded just before 7:20 a.m. New York time.

Law enforcement officials said the attacker, identified by police as Akayed Ullah, 27, chose the location because of its Christmas-themed posters, a motive that recalled strikes in Europe, and he told investigators that he set off his bomb in retaliation for U.S. airstrikes on Islamic State targets in Syria and elsewhere.

It was the third attack in New York City since September 2016, and the second in two months, coming only weeks after eight people were killed in a truck attack along a Hudson River bike path. Like the earlier two, the attack Monday appears to have been carried out by a “lone-wolf” terrorist.

The explosion Monday morning echoed through the subway tunnels just off Times Square and filled parts of the Port Authority Bus Terminal with smoke as commuters fled. Even as smoke still filled the chamber, Ullah was subdued by Port Authority police officers.

After he was subdued, Ullah was taken to Bellevue Hospital Center, where he was in serious condition with burns to his hands and abdomen, according to Daniel A. Nigro, commissioner of the New York Fire Department. Three other people had minor injuries, he said.

Ullah came to Brooklyn from Bangladesh through a visa program available to people who have relatives who are US citizens.

On Monday afternoon, in his first remarks on the attack, President Donald Trump assailed the current immigration system that allows for extended family members, and not just spouses or minor children, to receive green cards.

“The terrible harm that this flawed system inflicts on America’s security and economy has long been clear,” Trump said in a statement. “I am determined to improve our immigration system to put our country and our people first.”

The attack occurred in a long pedestrian walkway connecting the Eighth Avenue, Seventh Avenue and Broadway subway lines. Among the commuters traveling beneath Times Square was a man in a hooded sweatshirt. Then came a deafening boom — from him — and then smoke.

Everyone ran.

Ullah had attached the pipe bomb to himself with a “combination of Velcro and zip ties,” said James P. O’Neill, commissioner of the New York Police Department. It was crudely composed of a length of pipe stuffed with match heads, its ends stopped up. A broken Christmas tree light was the detonator: When lit, the filament ignited the match heads, the device powered by a 9-volt battery.

The explosion, captured on surveillance video, burned and cut Ullah, but because it did not detonate properly, it did not produce shrapnel, often the deadliest element of a pipe bomb.

“I think he was prepared to die, and we see him connect the wires on the video,” said a law enforcement official who spoke on the condition of anonymity because the assessment of the suspect’s actions was still preliminary.

As people streamed through the station, Officer Anthony Manfredini of the Port Authority Police Department rushed toward the smoke, said Robert Egbert, a spokesman for the main police union that represents Port Authority officers. A former marine, Manfredini, 28, found the suspect on the ground with “visible wires coming from his jacket into his pants,” Egbert said.

Sarah Maslin Nir/William K. Rashbaum/The New York Times