The NTSB held another media briefing Friday night following day two of their investigation into Tuesday's deadly explosion that killed Akil Drake, a Chase Bank employee and injured seven others at the Realty Building in Downtown Youngstown.

NTSB Board Member Tom Chapman says there are very serious concerns about the integrity and safety of the building.

"Access is not currently possible by us nor by anyone until appropriate determinations are made regarding its safety," Chapman said. "That determination is outside our expertise and is in the hands of city, county and state authorities."

Chapman said much of their first full day of the investigation was devoted to conducting interviews with the workers in the basement and vault of the building at the time of the incident. He added the NTSB investigation has an added level of difficulty because the federal agency is also not permitted in the building for safety concerns.

21 News has reported those workers were with GreenHeart Companies, hired by the city of Youngstown to relocate gas lines because it conflicted with city road work, as part of the SMART 2 project.

Chapman says at the time of the incident, four workers were on site. He says the crew was engaged in clearing out old piping and other outdated infrastructure and was unaware that one of the pipes in the vault was pressurized at the time. He says the crew also had no reason to believe gas was present in the pipe and they didn't smell gas during the day which indicates there was not an ongoing leak throughout the day.

"The crew was engaged to clear out old piping and other outdated infrastructure and debris from the basement and vault area which extends underneath the sidewalk adjacent to the building. The accident sequence began as the crew made two initial cuts into piping along the basement wall," Chapman said. "When a third cut was made, the crew immediately realized that there was a problem and that gas had been released. The work crew evacuated the basement, alerted the bank employees upstairs and pulled the fire alarm. We understand that at least one of the workers called 911."

Chapman praised the workers for alerting residents in the upstairs apartments and says they assisted in helping to evacuate residents. He says it was only 6 minutes later the explosion occurred. 

"The work crew evacuated the basement, alerted the bank employees upstairs, and pulled the fire alarm." Chapman said. "We understand at least one of the workers called 911."

The NTSB is not clear whether the building's fire alarm also sounded in Chase Bank.

"Based on today's interviews and other available information, the explosion occurred approximately six minutes after the pipe was cut," Chapman added.

When asked by 21 News Anchor Erin Simonek if the explosion, as it was described, was an accident, Chapman responded, "absolutely."

"There's nothing at this point to indicate that there was anything nefarious about what happened. All the indications are that it was very much an accident, " Chapman said.

Chapman says everyone involved is cooperating and their investigation will continue for several more days.

As the cause of the explosion is now clear, the NTSB told 21 News they are looking into how long the gas line has been abandoned and why it was not shut off.

Several questions were answered as concerns still linger over the future of Realty Building.