ST. PAUL — A fourth-grade field trip to a Mississippi River park popular with fossil hunters turned deadly Wednesday when gravel saturated by persistent rain gave way, killing one child and injuring two others. A fourth child was missing.

The landslide at Lilydale Regional Park swept over a group of children from Peter Hobart Elementary School in St. Louis Park, burying one to the waist and completely burying another, Fire Marshal Steve Zaccard said.

Another child suffered minor injuries in the landslide, which happened about 1:15 p.m. The three children were taken to Regions Hospital, and one died, Zaccard said. The child with minor injuries was treated and released, and the other child is in serious condition, Zaccard said.

The search proceeded cautiously for the fourth child as light rain fell throughout the afternoon. Zaccard described that search as a recovery effort.

Crews were trying to make the scene safe Wednesday night, using fire hoses to wash away loose dirt before it falls on searchers, Zaccard said. Digging had stopped earlier, and crews were setting up lights and preparing to search into the night, he said.

“Obviously we have a very tragic situation here today,” Mayor Chris Coleman said at a news conference hours after the gravel slide.

Police and firefighters dug frantically with shovels and their hands to get to the children, Zaccard said. He estimated it took an hour to rescue a child buried to the waist, and another half-hour to reach the buried child as more workers and equipment were moved in.

As news of the gravel slide broke, parents gathered at the school, located in a western Minneapolis suburb several miles away, to await their children.

Zaccard said several days of rain had softened the ground at the park, but it and nearby Cherokee Park were open and available to the public Wednesday. Brad Meyer, the city’s parks and recreation spokesman, said it’s rare to close a park “unless there is a major hazardous condition.”

“We didn’t have any indication that we should close the park or that it was unsafe,” Meyer said.

The area is popular for school field trips because students can hunt for fossils in the soft sandstone cliffs, said Michael Rutzick, who was at the park Wednesday and lives in nearby Mendota Heights.

“It’s all sandstone. Sandstone and trees,” Rutzick said.

Tammy Olson, who lives two blocks away, was at the scene with her two young nephews Wednesday. She said she never considered the park dangerous.

“I’ve been here six years, and my husband has been here 23. He said there’s never been an issue with accidents down there,” Olson said.

A firefighter also was injured by a falling rock but was expected to recover, Zaccard said. Another firefighter suffered a back injury, and both firefighters were back home, the fire department tweeted.