YOUNGSTOWN, Ohio — A crew working in the basement area of an Ohio building intentionally cut a gas line not knowing it was pressurized before a deadly explosion this week, the National Transportation Safety Board said Friday.

NTSB board member Tom Chapman said preliminary investigation shows workers were in the basement to clear out piping and other outdated infrastructure and debris from the basement and vault area — which extends underneath the sidewalk next to the building — in anticipation of a city project to fill in the area and replace the sidewalks. A crew of five people and a supervisor had been on site that day and four of the workers were there when it happened, he said.

“It was an apparently abandoned service line coming off the main, which ran parallel to the street,” Chapman said.

He said workers smelled no gas before they started cutting the pipe and knew there was a problem when they made the third cut.

At that point, workers pulled the fire alarm and alerted residents and bank employees to evacuate. Chapman said the explosion happened six minutes later. He also said all indications are that it was accidental.

Investigators will try to determine why the pipe was pressurized and how long it had been that way.

Chapman said the investigation would continue without access to the inside of the building until engineers can determine if the building is safe to enter. He said the NTSB has gotten security video from inside the bank and other video evidence.

The explosion Tuesday afternoon blew out much of the ground floor of Realty Tower, killing a bank employee and injuring several others. It collapsed part of the ground floor into its basement and sent the façade across the street. Bricks, glass and other debris littered the sidewalk outside the 13-story building, which had a Chase Bank branch at street level and apartments in upper floors.

Investigators are also trying to discern whether people in the bank heard the fire alarm.

Youngstown Mayor Jamael Tito Brown said in a news release Friday that the city had contracted with a construction company called GreenHeart to perform private utility relocation in the basement of the Realty Tower. He said “there is no evidence” that cutting the gas line the NTSB mentioned was necessary to complete that work.

Greenheart did not immediately respond to a request for comment Friday evening.

The bank employee, 27-year-old Akil Drake, had been seen inside the building just before the blast, police have said. Firefighters rescued others as they cleared the building.

Seven injured people were taken to a Youngstown hospital. One woman remained hospitalized as of Thursday in critical condition, but her name and further details on her injuries have not been disclosed. Three others were in stable condition, and the other three were released.