ST. PAUL — A rockslide that killed two children during a school field trip at a St. Paul park was the result of natural causes, and city officials could not have prevented or predicted the tragedy, according to findings of two investigations released Thursday.

Mayor Chris Coleman hired two independent investigators to examine the incident at Lilydale Regional Park. One investigation looked at what the city knew and whether the rockslide could have been prevented. The other examined the geotechnical aspects of the slide and what caused it.

The investigators found the collapse was an “unpredictable, natural occurrence,” the city said, adding it will continue to suspend permits for fossil hunting in the park while it creates a plan to keep people informed of potential risks.

“We can put up a sign. We can put up a fence. We can put up a gate. But we can’t keep people from getting back into the 17 miles of wild area along the bluffs of the river in the city of St. Paul,” Coleman said. “The best thing we can do is arm people with knowledge.”

Lilydale Regional Park runs along the Mississippi River. It’s a popular destination for fossil hunting, which requires a city permit.

Fourth-graders from Peter Hobart Elementary School in St. Louis Park were on a fossil-hunting field trip in the park on May 22 when a mass of sand and broken shale fell from a bluff overlooking the fossil beds near the East Clay Pit. Two students — 10-year-old Mohamed Fofana and 9-year-old Haysem Sani — were killed. Two others were injured.

As he announced the studies’ findings, Coleman said Thursday the victims’ families are in his thoughts and prayers.

“To those families, the tragedy will never end, and we need to always remember that,” he said.

One investigation led by Don Lewis, the dean at Hamline University’s School of Law, found the slope failure was apparently due to natural soil erosion common along the Mississippi River bluffs, aggravated by a rainy spring and foot traffic along unmarked trails. The other investigation, led by Ryan Benson of Northern Technologies, Inc., found the rockslide was a result of natural environmental changes, combined with ongoing impacts of weather on sand and shale.

Benson said there is no evidence that anything man-made triggered the rockslide. He added it’s hard to predict when, where, or how big the next rockslide would be, and the city should consider similar bluff areas to be a risk.

The team’s report said: “The only variable that is predictable with a high level of certainty is that slopes of this nature … are inherently unstable, will undergo additional weathering, and are highly likely to undergo failures of varying degrees in the future.”

Lewis’ investigation found the city was aware that slope failures had occurred within the park and along the river’s bluffs before. In May 2011, there was one failure north of the East Clay Pit, but it did not damage people or property. Nine months later, a Parks and Recreation forestry supervisor saw erosion while ice climbing in the Fossil Ground, and reported that the hillside was at risk.

Mohamed Bah, one of Mohamed Fofana’s uncles and president of the Guinea Association/Community of Minnesota, told the Star Tribune the results don’t make sense, and parents wouldn’t have sent their children to the park if they knew the dangers.

“If they had knowledge about the area that it was risky, why would they send vulnerable kids there?” Bah told the newspaper.

Lewis’ team found that while city officials knew soil erosion could be an environmental concern, there was no evidence that they knew of unstable bluffs that posed a safety risk.

“The city did not know soil erosion posed a threat,” Lewis said.

Sarah Thompson, a spokeswoman for St. Louis Park Public Schools, said the school is focusing on education, as well as recovery and healing for students.