MINNEAPOLIS (WCCO) — Federal response teams are in Minneapolis to begin sorting out what caused the deadly explosion and partially collapse at Minneapolis’ Minnehaha Academy Wednesday morning.

NTSB officials arrive at Minnehaha Academy (credit: CBS)

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The National Transportation Safety Board says the damage is so extensive, it has to wait to determine if the blast area is safe to enter before beginning their investigation.

They will gather factual information throughout the next few days.

Employees inside the private Christian school smelled gas Wednesday morning just before 10:30 a.m.

They were trying to get everyone out when the building exploded.

Investigators say it happened while crews were moving a gas meter.

No students were hurt, but two employees who touched nearly everyone in the school were killed.

Receptionist Ruth Berg died when the part of the building she was in collapsed.

(credit: CBS)

Searchers found a man’s body in the rubble at about 8 p.m. Wednesday night, who Minnehaha Academy officials later confirmed was custodian John Carlson.

The question everyone wants answered is how did this happen?

As mourners left bouquets of flowers near the scene of Wednesday’s tragedy, the search for answers — which will be slow and meticulous — got into full swing.

With all victims recovered from the debris, this is now an evidence scene.

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“We understand that this explosion happened in the course of moving a gas meter,” said Christopher Hart of the NTSB. “And that’s what we’ll be exploring starting [Friday].”

Anoka Attorney Fred Soucie has represented many victims of past gas explosions.

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“If gas escapes from a system, somebody screwed up, and that’s all there is to it,” Soucie said.

An explosion in Ramsey in 2004 killed three people and seriously injured another.

Soucie says containment of natural gas is the critical factor to preventing these tragedies.

Minnehaha Academy before and after the explosion (credit: Google Earth/CBS)

“The principle is that you do not let the lion escape. You just don’t,” he said. “If you do let the lion escape, then you’re at fault for letting it escape.”

Investigators will probe for the source of the gas leak and the point of ignition.

Contractors were replacing a gas meter and piping at the school, though it is still unclear what CenterPoint Energy’s role might have been, or why work was done without the gas shut off.

“You must reassemble every single piece of the line, every single piece of equipment anywhere near the line, and never, ever jump to any kind of a conclusion no matter what,” Soucie said.

The NTSB is leading up the probe. A blast with this sort of complexity could take more than a year to come up with definitive answers and issue a report.

Three of people hurt in Wednesday’s explosion remain in the hospital. Two are in satisfactory condition.

WCCO is told Minnehaha Academy employee Brian Duffey is still in critical condition at Hennepin County Medical Center, and his family says he will require additional surgeries. Duffey is an assistant soccer coach.

One other victim is also at HCMC in satisfactory condition as of Thursday night.

In all, 10 people were hurt in the explosion. Nine went to the hospital.

Since school is not yet in session, only administrators and a few others were inside the building at the time of the blast.

Principal Donna Harris says Minnehaha Academy’s Lower and Middle school — which is located at a separate site than the damaged Upper School — will begin classes on Aug. 23.

She says they are still searching for a site for Upper School students, but classes for them are tentatively set to begin on Tuesday, Sept. 5.

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A GoFundMe page has been set up to help rebuild the Upper School.