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

The blast occurred near Central Square on Tuesday afternoon and impacted a building that contains a Chase bank and apartments. One person was killed and seven others injured in the explosion, officials 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 prior to the explosion on Friday.

The preliminary investigation suggests that the workers were in the building to clear out old utility infrastructure from the basement and vault area, including old piping, as part of a city project to fill the vault area and replace the sidewalks above, according to NTSB board member Tom Chapman.

The workers made two initial cuts into piping along the basement wall, he said. After making a third cut, "The crew immediately realized that there was a problem and the gas had been released," Chapman said during a press briefing on Friday.

The workers quickly evacuated the basement and notified bank employees of the gas leak, Chapman said. They pulled the fire alarm and at least one worker called 911, he said. The workers were also "instrumental" in helping evacuate residents in the apartments, he said.

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

No gas was smelled during the day as the crew was working in the basement, indicating that there wasn't an ongoing leak, Chapman said.

"The crew was unaware that one of the pipes in the vault was pressurized at the time. Nor did the work crew have reason to believe gas was present in the pipe," Chapman said.

The NTSB had said on Thursday that a cut to the pressurized service line is a "central focus" of their investigation to determine the cause of the gas release and explosion.

Following the explosion, the Public Utilities Commission of Ohio and Enbridge Gas, the service provider for the area, had gained access to the basement and discovered the cut to the pressurized but inactive below-ground service line, Chapman said.

Chapman said there is no evidence to suggest anything "nefarious" in the incident.

"All indications are it was very much an accident," he said.
 

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

"Part of what we'll be looking at is what are the proper procedures and were those proper procedures followed," he said.

Chapman called the damage in 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, officials said.

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

The building and a neighboring hotel have closed and streets in the surrounding area are shut off to traffic indefinitely due to the potential for structural collapse, authorities said.

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

The NTSB’s investigation is expected to last approximately one week, with a preliminary report anticipated in about 30 days. Final reports typically take between 12 and 24 months to complete.