A fiery explosion destroyed three buildings and ignited a massive blaze in downtown Manhattan, officials and witnesses said Thursday afternoon.

Nineteen people in all were injured, officials said. Four were transported to local hospitals in critical condition, including two with serious burns and another who was unconscious, the Fire Department of New York said. Several people were treated at the scene. The 19 included five service members — firefighters and other emergency responders — who suffered non-life threatening injuries, the FDNY said.

MAN 7-ALARM 125 2 AVE, MIXED OCCUPANCY MAJOR BUILDING COLLAPSE, — FDNY (@FDNY) March 26, 2015

Now #FDNY operating on-scene at 123 2nd Avenue in Manhattan. pic.twitter.com/bg8vx1TwLX — FDNY (@FDNY) March 26, 2015

New York City Mayor Bill de Blasio said at a press conference that the explosion occurred at a building on Second Avenue in Manhattan’s East Village, causing one building to partially collapse and the one next to it to collapse completely. By Thursday night, three of the four buildings engulfed by the flames were destroyed as the fire continued to burn, according to officials.

The FDNY said it sent 250 members to the scene for what quickly escalated into a seven-alarm fire by 4 p.m. ET.

FDNY Commissioner Daniel Nigro said firefighters “certainly didn’t expect” the severity of the unfolding scene and that the explosion “blew the front of (the building) across the street.”

Mayor de Blasio said preliminary evidence showed the explosion was likely “gas related” and was caused by possible plumbing and gas work occurring inside one of the buildings.

The mayor said there was no indication that either 911 or New York energy company Con Edison had received calls complaining of a gas leak on Thursday. Con Edison said in a statement to NBC New York that inspectors had been at the site of the explosion at 2 p.m., one hour before the blast occurred, evaluating a meter installation for a gas service upgrade. Con Edison found that the work done by private contractors in the building did not pass inspection and gave instructions for necessary changes.

“The work failed our inspection for several reasons, including insufficient spacing for the installation of the meter in the basement,” the company said in the statement.

Con Edison said there were no reports of gas odors in the area prior to explosion and “a survey conducted yesterday of the gas mains on the block found no leaks.”

“Until we know what happened here we cannot pass judgment,” Mayor de Blasio said of the incident at the press conference. “We need to fully investigate.”

Because of the copious amount of smoke, Office of Emergency Management Commissioner Joe Esposito advised nearby residents to “keep your windows closed and limit your time outside as much as possible.”

A photo posted by the FDNY appeared to show one building fully collapsed, with another partially collapsed at the ground level while firefighters worked to put out huge billows of smoke and flames.

Nolan Hayes, who was at a nearby Starbucks at the time, told NBC New York he heard a “very loud” sound when the building collapsed.

“We walked outside and noticed a lot more smoke than we expected … You look outside and see no building being there at all,” he said.

Initial video and photos posted to social media showed huge flames shooting out of the front of the building, which appeared to house a restaurant on the ground floor, and thick plumes of white smoke billowing from the building, NBC New York reported.

“I heard a really loud bang,” Ariel Setear, who lives two blocks away, told NBC News. “I thought maybe it was thunder or a car being hit, but it sounded bigger than that … I couldn’t see any fire.”

“And then the flames came up, they were huge,” Setear said. “The flames were maybe two stories.”

A video posted by Monique Roberts (@mo225nique) on Mar 26, 2015 at 12:44pm PDT

Aftermath. All media: please feel free to use my shots of this, credit if possible. pic.twitter.com/3qeEjJKnb2 — Perry Kroll (@perrykroll) March 26, 2015

#BREAKING: Aerial view of building collapse @NYPD9Pct. Numerous rescue units on scene #SOD #ESU #K9 #Aviation #FD pic.twitter.com/d9iX4GOBUd — NYPD Special Ops (@NYPDSpecialops) March 26, 2015

A photo posted by Vincent Li (@videovincent) on Mar 26, 2015 at 12:31pm PDT

OMG POMME frites blew up pic.twitter.com/YQAXBUJalt — Vincent Li (@vincentli2008) March 26, 2015

A photo posted by Monique Roberts (@mo225nique) on Mar 26, 2015 at 12:36pm PDT

This article originally appeared at NBCNews.com

