A massive explosion blew out much of the ground floor of an apartment building, killing a bank employee and injuring several other people.

Police and emergency officials initially blamed natural gas, but the fire chief later said the cause is under investigation.

The blast happened around 2.45pm Tuesday, collapsing part of the ground floor of Realty Tower into its basement and sending the façade across a street where both sides had been blocked off by orange construction fencing.

The 13-storey building has a Chase Bank branch at street level and apartments on upper floors.

The bank employee was a 27-year-old man who had been seen inside the building right before the blast, the Youngstown Police Department announced early on Wednesday.

Youngstown Fire Chief Barry Finley said in a news conference on Tuesday that firefighters rescued several people and cleared the building to ensure no one else was hurt.

JPMorgan Chase mourned the loss of a colleague and said it would work with local officials.

“Our hearts go out to their family as well as our injured employees, their families and others affected by this tragedy,” a company statement read.

Police and the Mahoning County Emergency Management Agency said there was a natural gas explosion, but the fire chief later said it was too early to say.

“We have no idea what caused the explosion. We know that there was an explosion and it did a lot of damage to the bottom of the building,” Mr Finley said on Tuesday.

He said none of the apartments in the building are habitable.

Seven injured people were taken to Mercy Health Hospital in Youngstown.

A hospital spokesperson said one was in critical condition.

The blast shook downtown Youngstown, a city of about 60,000 residents. Bricks, glass and other debris littered the sidewalk.