“She was clearly very frazzled,” said James Cole, 24, who works in advertising and went to the scene after hearing the blast from his apartment a few blocks away. A man in the crowd climbed up the escape’s ladder, Mr. Cole said, and helped the woman down.

Faisal Jamil, 37, watched as two people jumped down from a fire escape ladder when it would not deploy, falling about 10 feet onto the pavement below. One of the jumpers, a man, landed on the sidewalk, then began to shout, “ ‘Oh my God, we have to save more people,’ ” Mr. Jamil said.

A different man ascended the ladder, climbing up to the higher floors as smoke seethed from the building’s base. He knocked on windows, calling out for residents to evacuate. People below screamed for the man to climb back down as the fire grew.

“It turned into pretty dramatic flames, 30 or 40 feet above the building,” Mr. Cole said.

Moments before the explosion, Blake Farber, 29, a film director, was walking down the street when he felt the percussive blast. He turned back to see the building blown open and people fleeing from Sushi Park, the popular restaurant at 121 Second Avenue. A man was crawling on the sidewalk, his face covered in blood.