Fire crews in St. Paul worked until early Saturday morning to dig out the bodies of two workers who died on a Highland Park construction site after a trench collapsed on top of them, in what authorities were describing as a tragic accident.

Search teams recovered the second body at 2:30 a.m. Saturday, about 12 hours after loose dirt surrounding the trench created a "dirt avalanche" that likely killed the two construction workers immediately, said St. Paul Deputy Fire Chief Roy Mokosso. The body was found 9 feet underground.

The other body, which was recovered Friday night, was partially protruding through the dirt. But the person's injuries were "incompatible with life" when firefighters arrived, Mokosso said.

The Ramsey County Medical Examiner's Office had not identified the victims as of Saturday night. Mokosso said police informed the families of the victims on Friday night.

Firefighters from St. Paul's Rescue 3 rig were still at the scene Saturday afternoon on Mount Curve Boulevard near Pinehurst Avenue. Someone had left a bouquet of flowers across from the trench, which was braced open with rescue equipment.

Mokosso said three contractors for a private construction company had been working on what appeared to be a water or sewage pipe project when the accident occurred. The trench is adjacent to an apartment building under construction.

A trench box — a device designed to prevent cave-ins — was sitting next to the site, and Mokosso confirmed the workers were not using it when the trench collapsed.

A third construction worker on the site "quickly tried to get in and help the individuals out, but realized pretty quickly there was little they could do," he said. That worker called 911 at 2:40 p.m.

Firefighters arrived quickly, but the rescue operation shortly turned into a body recovery mission. No first responders were injured during the operation.

It was unclear what set the collapse into motion, but the website of the Centers for Disease Control and Prevention warns that "workers should never enter a trench that does not have a protective system in place."

The CDC recorded 373 trench fatalities from 2003 to 2017, with more than 80% occurring in the construction industry. Dirt is deceptively heavy — a cubic yard can weigh as much as a compact car — which is why the CDC says most collapses are not survivable.

"It's similar to drowning, but worse because of the weight," said Mokosso.

To recover the bodies, St. Paul and Minneapolis fire crews used a high-velocity "air knife" device and a vacuum truck to break through the heavy dirt, Mokosso said.

"Trench operations are very dangerous," he said, so the rescue teams had to brace the hole as they dug it open. It took about six hours to recover the first body.

"If they're 6 feet down, you can't just jump down in there," Mokosso said. "It's a slow, laborious process."