Fire officials say 19 people are injured, four in critical condition, following an explosion and building collapses in the East Village.

Mayor Bill de Blasio says 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. at 121 Second Ave., the mayor says.

The president of Con Ed says 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 the mayor says 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 buildings at 119, 123 and 125 Second Avenue.

Fire officials say the explosion and fire caused complete collapses of the buildings at 119 Second Ave., 121 Second Ave. and 123 Second Ave.

The FDNY says they were still fighting pockets of fire as they cleared debris as of early Friday morning.

Fire Commissioner Daniel Nigro says the fire spread to seven alarms, with 250 personnel responding. 

He says the explosion blew the front of 121 Second Avenue across the street.

Nigro says firefighters made "extremely dangerous" searches of the buildings in the 15 minutes before they started to collapse.

He says the department expects to remain on the scene for some time.

De Blasio says most of the injuries apparently occurred at the moment of the blast, not in the collapse of the buildings, which occurred after the fire raged.

The Office of Emergency Management says there are no reports of missing people from the fire.

An official with Bellevue Hospital told NY1 that they took in three patients with injuries related to the explosion. They said two were in serious condition and one was in fair condition.

The Mount Sinai Health System says there are five patients with non-life-threatening injuries at Mount Sinai Beth Israel.

Officials stressed that the investigation is still developing, but Nigro said that "To the best of our knowledge, they were working on the gas in the building."

The Red Cross set up a reception center for those who lived in the four buildings affected by the blast at P.S. 63, located at 121 East Third Street.

According to City Councilwoman Rosie Mendez, 11 buildings in the area have evacuation orders. She said that includes 144 apartments.

Mendez says 80 people - 79 adults and one baby - have registered with the Red Cross. She says 30 people - 29 adults and one baby - need a place to stay.