YOUNGSTOWN, Ohio (AP) — 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NTSB board member Tom Chapman said workers were in the basement to clear out piping and other outdated infrastructure. 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 after that cut. Investigators will try to determine why the pipe was pressurized.

The explosion Tuesday afternoon blew out much of the ground floor of Realty Tower, killing a bank employee and injuring several other people. 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.