"I'm not ready to call it a recovery yet", Fruetel told reporters at a 4:30 p.m press conference.
One other person, 82-year-old school janitor John Carlson, is still missing after Wednesday morning's blast.
The blast hit the Minnehaha Academy School on the 3100 block of West River Parkway, the fire department said on Twitter. HCMC tweeted at 9:19 p.m. that they are now treating four people - one is critical and three are in satisfactory condition.
"At least three of the injured were initially listed in critical condition and four others in serious condition following the blast".
Master Mechanical Inc. was issued a permit on June 7 for "gas piping and hooking up meter" at the location, the paper reported, citing city records.
A "Humans of Haha" story in the school newspaper previous year profiled Carlson, who was known for handing out Dilly bars to students and always saying "hi" in the hallways.
"It could have been a lot worse", Dr. Jim Miner, Hennepin County Medical Center's chief of emergency services said during a media briefing.
The explosion, which happened during construction work on the gas supply, ripped Minnehaha Academy's Upper School in half about 10am.
Minnehaha Academy President Donna Harris survived the explosion.
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Activities at the building have reportedly resumed after a brief evacuation following the incident. The defendants managed to break free and take possession of the guard officers' weapons.
The Christian school serves students from pre-kindergarten through to year 12.
"We heard a man yell 'gas!' and 'get out!' and we saw one sprint down the street and one sprint into the building, kind of standing in the doorway, yelling it", he told WCCO.
"Ryan Larsen, a financial officer with the company, said, 'We've got people on the site there". "We're going to do our best to locate everyone that's unaccounted for".
"All the windows just kind of burst out, and there was a huge explosion that was so loud it kind of shook your insides", student Kylee Kassebaum told the Star Tribune.
The explosion occurred in the high school building of the private K-12 school.
Gov. Mark Dayton released a statement saying his office was in contact with city officials and the state "will provide any and all resources necessary" to help first responders and ensure everyone is safe.
A woman who identified herself as Berg's sister, and who lives in Idaho, wrote on Facebook that she is "trying to hold it together".
A utility-boiler room was the apparent epicenter of the explosion. As soon as I turned the corner, you could see the daylight, and I knew, you know, there's something wrong'. The explosion affected only the upper school; the lower and middle school campus is about a mile and a half away.
The blast knocked him off his feet, "and then it was just kinda chaos from there", he said.