NTSB investigators on scene in Minneapolis to investigate deadly school explosion

— -- National Transportation Safety Board investigators are on scene in Minneapolis to investigate the explosion that caused a school building to collapse Wednesday, killing two staff members and injuring several others, an NTSB official said this afternoon.

The Minneapolis Fire Department said the explosion at Minnehaha Academy was caused by a gas leak and said contractors were doing construction on the gas lines at the time.

NTSB investigators arrived today and Friday will be their first full day on scene, NTSB board member Christopher Hart said. Investigators expect to be on scene for five to seven days.

The mission is to understand what happened and why it happened, Hart said, so investigators can make recommendations to keep this from happening again.

The probable cause of the accident will not be determined on scene, Hart said, adding that it usually takes a year to complete the final report.

Witnesses are asked to contact the NTSB.

Minnehaha Academy said the two individuals killed Wednesday were staff members John Carlson and Ruth Berg.

Two people injured in the incident remain in the hospital this afternoon, one in critical but stable condition and one in satisfactory condition, according to the hospital.

Officials said it was fortunate the incident did not occur during the school year while students were in class.

One father told ABC affiliate KSTP he was in the school counselor’s office with his daughter and wife at the time of the explosion. He said someone came into the office and told them there was a gas smell and everyone needed to get out.

Seconds after his daughter began to exit, there was a "huge explosion," he told KSTP. "It was a large, huge, 'boom.' One of those movie scenes where you kind of start shaking and it knocks you off your feet. It was pretty intense.”

“Stuff started falling out of the ceiling. Kind of rocked my daughter and she fell back into the building," he explained. "From there, people were in panic. We heard a lot of screaming and the glass blew out in the office."

They exited the building out a back door, and that’s when he said he realized the severity of the situation. He ran back inside to help others but some parts of the building were completely blocked by debris, he said.

His wife and daughter were scratched up a little bit, but were otherwise OK, he shared.

“We're blessed," he said. "This could’ve been a lot worse.”

ABC News' Jeffrey Cook contributed to this report.