An explosion at New Haven Chlora-Alkali, H. Krevit & Company blew part of the roof off the chemical plant at 73 Welton St. in New Haven and neighbors were evacuated for a couple of hours.

The incident, which happened around 11:30 a.m., was initially called in as a hydrogen-cyanide explosion, but officials later said it was actually an explosion in a natural gas feeder line to an overhead heater at the plant, according to the public information officer.

Now officials are backing away from the natural gas theory and said they found two hydrogen tanks with the tops blown off and said they appeared to have exploded under pressure.

The damage at the plant is extensive and part of the roof is gone, but no one has been injured, according to officials.

The explosion also blew out windows of a home nearby.

Residents in the area said they had no idea what was happening.

"They were vibrating back and forth from the compression of the air," Dominic Ferrara, of New Haven, said.

"The walls shook everything. The whole building shook. I thought the TV was going to fall off the wall," Donna Maine, of New Haven, said.

The explosion also sent debris onto the Amtrak rails, but the tracks have reopened.

Residents in the block around the company were evacuated, but have been allowed to return to their homes.

Residents from as far away as Guilford and Madison reported feeling the explosion.

Bill Lacey was at the company to service the elevator when the explosion happened and saw it happen.

“My van shook and I turned around and when I looked the top of the roof was about 40 feet in the air and insulation and foam was flying all over the place and like a white puff of smoke came out and that was about it,” Lacey said.

He said he was taking parts out of his van during the explosion.

“It was pretty scary,” he said.

Hazmat crews responded to the scene as a precaution in the event of another explosion, but most emergency crews have now left, other than investigators.

The New Haven Fire Department is investigating.

“It’s indicative of pressure more than anything else. It wasn’t human error. It appeared to more of operational error within the equipment itself,” Assistant Fire Chief, Ben Vargas, said.

Crews from the state Department of Energy and Environmental Protection are removing all the chemicals from the building and authorities said the streets surrounding the facility will remain closed until the work is complete.

A specialist was also called in to test air samples.

The company where the explosion occurred was created in 1919 as a manufacturer of Sodium Hypochloride bleach and it was sold in 2016. The company continues to makes Sodium Hypochloride, as well as a line of other chemical products and services companies in the metal finishing, swimming pool and water treatment industries.