The National Transportation Safety Board said an inactive but still pressurized gas line was shut off six minutes before a devastating explosion rocked downtown Youngstown, Ohio.

The explosion occurred near Central Square on Tuesday afternoon and hit a building containing a Chase bank and apartments. One person was killed and seven others were injured in the explosion, authorities said.

The NTSB sent a team of pipeline and hazardous materials investigators to Youngstown to investigate the natural gas explosion and were able to speak with workers who were in the basement of the building before the explosion on Friday.

Preliminary investigation suggests workers were at the building to clean out old utility infrastructure in the basement and vault area, including old pipes, as part of a city project to fill the vault area and replace the sidewalks above, according to an NTSB board member. Tom Chapman.

Workers made two initial cuts in the pipes along the basement wall, he said. After making a third cut, “the crew immediately realized there was a problem and the gas had been released,” Chapman said during a news conference Friday.

Workers quickly evacuated the basement and notified bank employees about the gas leak, Chapman said. They pulled the fire alarm and at least one worker called 911, he said. Workers were also “decisive” in the evacuation of apartment residents, he said.

The explosion occurred approximately six minutes after the outage, Chapman said.

No gas was smelled during the day while the crew was working in the basement, indicating there was no leak in progress, Chapman said.

“The crew did not know that one of the pipes in the vault was pressurized at the time. The crew also had no reason to believe there was gas in the pipe,” Chapman said.

The NTSB had said Thursday that a break in the pressurized service line is a “central focus” of its investigation to determine the cause of the gas leak and explosion.

After the explosion, the Ohio Public Utilities Commission and Enbridge Gas, the area’s utility provider, gained access to the basement and discovered the break in the pressurized but inactive underground service line, Chapman said.

Chapman said there is no evidence to suggest anything “nefarious” about the incident.

“Everything indicates that it was largely an accident,” he said.

The investigation will look into why that apparently abandoned service line was still pressurized and for how long, he said.

“Part of what we’ll look at is what the proper procedures are and whether those proper procedures were followed,” he said.

Chapman called the damage from the explosion “stunning” and said NTSB investigators have been unable to access the building due to concerns about its structural integrity.

“The damage to the building is devastating,” he said.

The floor collapsed into the basement, which was flooded, authorities said.

The body of a man who was a bank employee, identified as Akil Drake, was recovered from the basement early Wednesday, authorities said.

The building and a neighboring hotel have closed and surrounding streets are closed to traffic indefinitely due to the potential for structural collapse, officials said.

Chapman said the NTSB received cloud-based video from Chase on Friday that will help them understand where bank employees were at the time of the explosion.

The NTSB investigation is expected to last about a week, with a preliminary report expected in about 30 days. Final reports typically take 12 to 24 months to complete.