In a statement, the academy said earlier the explosion involved a natural gas leak, but it only affected the upper school.
Fruetel said the bodies were located close together.
Police said five people were taken to a hospital with injuries, though one has since been released.
"I used to live in San Francisco and there are two types: one is a short quick one the other is a longer one it felt like the plates were shifting".
A third person believed to be missing was found uninjured outside the building on the campus of Minnehaha Academy at about noon local time, he said.
"He wasn't scheduled to work today but he came in anyway", said the friend, Dave Swanson, who stood watch during the search on behalf of Carlson's wife, who couldn't be there.
Previously, two people were missing, according to assistant fire chief Brian Tyner.
The NTSB, the United States' premier civil transportation investigative agency, said it will investigate the explosion.
The upper school campus was minimally staffed for the summer, with year-round employees, girls on campus for summer cross-country practice and some school basketball and soccer players, according to the school.More news: North Korea ICBM Failed, But US Still Hypes Threat
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According to officials, the explosion was caused by a gas leak while contractors were working in the area.
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. The company said it was grateful to the first responders and bystanders who came to the aid of all the injured.
The fire department says both individuals worked at the school.
"It is a good thing that it is summertime", Asst.
All parts of the school that hadn't collapsed were inspected and cleared, Tyner said. "He could tell you stories about the buildings, about people". Part of the brick building collapsed after the explosion, authorities said. "Fire was going. And it's like, 'we're not going back until we get this guy out of here.' And got him out, and they got him on a stretcher". Two floors of the school collapsed into a sub-basement that quickly filled with water from firefighters. "She welcomed everyone with a smile", the school said on Facebook.
"We're going to get through it", Minnehaha Academy President Donna M. Harris said at the vigil.
Student Brett Allen said, "A guy was running out, like he say, shouting, 'Gas!' Just trying to get away". Some say their power flickered briefly afterwards.
"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 victims reportedly suffered injuries ranging from head wounds and broken bones to cuts from debris. He said that rubble is being combed for the pair but they could also be "walking around out here".