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 arrived in Youngstown on Wednesday night, and would announce more details at a news briefing on Thursday.

Police and emergency officials initially blamed natural gas, but the fire chief later said the cause remained under investigation.

“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.

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.