The explosion that injured four and led to an evacuation was caused by an "improvised low-tech explosive device" that was detonated in the 42nd Street passageway between 7th and 8th avenues, NBC News reported.
The New York Police Department says it is responding to a report of an explosion near Times Square. The A, C and E line are being evacuated at this time.
Ullah suffered the worst injuries but three others sustained minor injuries related to the explosion, including ear ringing and severe headache, Deadline reported. He was carrying a homemade explosive device, officials said.
"There are also no credible and specific threats against New York City at this time", New York City Mayor Bill de Blasio said Monday morning. The device, a pipe bomb, was attached to Ullah with Velcro and zip ties, said John Miller, NYPD deputy commissioner of intelligence and counterterrorism.
Armed police and fire crews reacted in force, with all trains from 42nd Street diverted and roads closed to traffic.
The bus terminal reopened around 10:20 a.m., according to the NYPD.
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"Could have been a lot worse", a federal law enforcement source told CNN. According to O'Neill, police are trying to determine if the hallway is where Ullah meant to detonate the device.
"Today's attempted mass murder attack in New York City - the second terror attack in New York in the last two months - once again highlights the urgent need for Congress to enact legislative reforms to protect the American people", Trump said in a statement.
Alicja Wlodkowski, 51, said that she had been in a restaurant inside the Port Authority when she suddenly saw a crowd of people running.
Cuomo and de Blasio are on the scene and have been briefed by law enforcement.
The incident prompted the closure of segments of New York's transit system and officials are urging people to stay vigilant and report any suspicious activity.
It was the deadliest terror attack in New York City since the September 11, 2001 attack on the World Trade Center.