More than 250 firefighters responded to what became a seven-alarm fire.

Among the 19 people reported injured, four were critically injured; four were firefighters who were taken to hospitals for treatment of minor injuries; and three others, including one emergency medical worker, declined medical attention at the scene, according to the Fire Department.

Nicholas Figueroa, 23, was in the restaurant in the building where the explosion occurred and was still missing early Friday morning, said his brother Tyler, 19. He said his family had searched the city’s hospitals and sought help from the police but had been unable to locate him.

Mr. de Blasio said workers from the utility Consolidated Edison had been on the site about an hour before the explosion and had found the work being done there to be deficient. But he said there were no calls reporting gas leaks before the explosion.

Michael S. Clendenin, a Con Edison spokesman, said several buildings on Second Avenue had been “undergoing renovations” since as far back as August.

“Based on records here, the building has had some work done inside; new gas service pipes; a lot of things; piping and such,” Mr. Clendenin said.

Image A Facebook photo of Nicholas Figueroa, who was missing as of early Friday morning.

The first reports the Fire Department received were for a building collapse, and witnesses described a frantic scene, with residents of the buildings scrambling down fire escapes to escape the raging flames and others dashing out of the rubble as the walls collapsed around them.

Niraj Desai, 36, a video game animator who works nearby, said he saw one woman stuck on a fire escape struggling to get the ladder unhooked.

“This poor girl was stuck upstairs,” Mr. Desai said. “She was having a hard time. You could tell there was a lot of emotion going on. A bunch of guys were down at the bottom helping her get down.”

Another man also made his way down the fire escape ladder before smoke started to pour from the building, he said.

“It was pretty crazy, pretty fast,” Mr. Desai said.

The authorities said that those who were seriously injured seemed to have been hurt in the explosion.

Image An explosion rocked the East Village on Thursday afternoon. The blast occurred at Second Avenue and East Seventh Street. Credit... Joseph Burgess/The New York Times

Residents of five buildings, including the four damaged or destroyed, were unable to return home on Thursday night, according to the authorities. The Red Cross established a temporary shelter for them at Public School 63, at 121 East Third Street. By 9 p.m., 55 adults and one child had registered for services there.

Blake Farber, 29, a film director, said he had smelled a lot of gas and had watched as two people who appeared to be working at the site were running between Sushi Park, a restaurant on the ground floor of 121 Second Avenue, and the residential entrance. Seconds later, he said, he felt the blast and was enveloped by dust and smoke.

“And then I saw a bunch of people running out of the restaurant — I saw a man crawling on the ground,” he said. “He was crawling and he turned around and his face was bloody.”

The blast happened just over a year after a gas explosion leveled two buildings on Park Avenue in East Harlem, killing eight people. Federal investigators found a crack in a Con Ed gas main near the site and a separation between that main and a smaller pipe.

The National Transportation Safety Board has not yet released its final report, which will provide its conclusions about what caused the explosion last March.