NEW YORK CITY -- Fire officials in Manhattan said, in a Thursday night update, that 19 people were injured in the explosion and building collapse in the East Village, and the four people are listed in critical condition.

New York City Mayor Bill de Blasio said that preliminary indications are that the blast was "gas-related." Private contractors had been doing plumbing work in the building just before the explosion, which occurred at 3:17 p.m. Thursday at 121 Second Avenue.

According to the president of Con Ed, the utility's inspectors had been at the building at about 2 p.m. checking out the installation of a gas meter but the "installation did not pass inspection." The Con Ed inspectors then left the scene.

People who were in the area at the time of the blast have posted on social media that they smelled gas just before the explosion but de Blasio said there are no reports of anyone phoning the city's 911 emergency line to report any gas odors.  

According to de Blasio, a fire that followed the explosion spread to neighboring buildings. The FDNY, which had more than 250 personnel responding, stated they were still fighting pockets of fire as of 9:30 p.m. Thursday. A spokesman said firefighters made "extremely dangerous" searches of the buildings in the 15 minutes before they started to collapse.

De Blasio says most of the injuries apparently occurred at the moment of the blast, not in the collapse and partial collapse of the two buildings, which occurred after the seven-alarm fire raged.