The National Transportation Safety Board is now leading the investigation into a massive explosion that blew out much of the ground floor of an apartment building in Ohio this week, killing a bank employee and injuring several other people.

The NTSB said pipeline and hazardous materials investigators were at the scene in Youngstown on Thursday and would announce details at an afternoon briefing.

Police and emergency officials initially blamed natural gas, but the fire chief later said the cause remained under investigation. The NTSB is leading the investigation because pipelines are considered a mode of transportation.

“We have no idea what caused the explosion. We know that there was an explosion and it did a lot of damage to the bottom of the building,” Youngstown Fire Chief Barry Finley said.

The explosion shook downtown Youngstown around 2:45 p.m. Tuesday, collapsing part of the ground floor of Realty Tower into its basement and sending the façade across a street where both sides had been blocked off by orange construction barriers. 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.

First responders were on the scene within minutes, and some of the initial search and rescue was recorded by a Mahoning County deputy’s bodycam video.

“We’ve got civilians over here screaming for help,” the deputy yells into his radio as he runs up to the building. “Be advised, there are people trapped.”

Moments later, the deputy helps arriving firefighters lower a ladder into a gaping hole where parts of the ground floor fell into the basement, and then helps a firefighter carry a woman to safety.

Before the partial video released by the sheriff ends, the deputy can be heard warning other responders that natural gas was leaking from an area near the front of the building.

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

Seven injured people were taken to Mercy Health Hospital in Youngstown. One woman was hospitalized 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.