Emergency crews in rural Denver, N.C., discovered the bodies of two young children who were killed on Sunday when a wall of dirt collapsed on them.

Dion Burleson, a spokesman for Lincoln County Emergency Management, told the Charlotte Observer that crews were making progress in recovering the bodies of the two children, but were exercising caution to avoid another cave-in.

Authorities said the children were buried under nearly 15 feet of fallen debris, in an excavated hole that measured 20 by 24 feet and was 24 feet deep. A rescue worker said the first body, a 6-year-old girl, was discovered Monday around 6 a.m. The body of a 7-year-old boy was found an hour later.

Family members and friends of both victims stood by as relief workers searched for the children.

The first 911 call was made at 5:07 p.m. on Sunday by a man identified as Jordan Caldwell, who is believed to be the boy's father. During the call, Caldwell informed police that the children were playing near the hole when he saw the dirt envelop them. About 65 rescue workers arrived on the scene to help dig out the children.

“We worked as if they were alive,” Burleson said. "Times like these are very difficult. It's devastating for the family and each and every responder." By 9:30 p.m., he conceded that the attempted rescue had become a recovery operation.

“It’s a tragic scene,” Charlotte Fire Capt. Rob Brisley, said.

The sheriff's department said that the two children had been playing late Sunday afternoon while the boy's father operated a backhoe nearby. Local Denver crews have since erected a safety barricade around the hole.

Burleson said an investigation into the collapse is under way.