(CNN) -- A field trip turned tragic for a group of fourth-graders in Minnesota when gravel drenched by rain gave way, killing two children and injuring two others.
Crews had worked late into the night Wednesday at Lilydale Regional Park, washing away soil with fire hoses and bringing in search dogs to look for one of the children who had remained missing overnight. His body was found Thursday morning, authorities said.
"Such an unbelievably tragic situation at #lilydale," St. Paul Mayor Chris Coleman tweeted Wednesday evening.
The children were students at Peter Hobart Elementary School who were at the park by the Mississippi River on a field trip.
Neighbors told CNN affiliate KARE that students often go to the area to look for fossils. There have been been occasions, they said, where parts of the ground moved or collapsed because of the wet conditions.
"It appears they were walking along in the ground (and) after the rain we've had, it was so soft and it gave way," St. Paul Fire Marshal Steve Zaccard told reporters. "They fell into what became a hole and the earth came on top of them."
Crews began a frantic search, digging with their shovels and hands to reach the children.
"They grabbed shovels out of the shed and they said that they needed shovels," resident Sue Zoccoli told CNN affiliate KARE.
The first child was pulled out quickly. But it took crews more than an hour to reach the second who was half submerged in water, reported affiliate WCCO.
"That's when they uncovered the hand of the third who later died," Zaccard said.
Two firefighters suffered minor injuries during the rescue: one was struck on the head by a rock; the other received a back injury.