Two Riverdale, boys who were severely burned Tuesday while playing inside a closed Pepco power substation remained in critical and serious condition yesterday at Children's National Medical Center.

Hospital officials said that Andrew Canter, 12, had second- and third-degree burns oer 60 percent of his body and was critically ill, although his condition had stabilized overnight. Shell Estrada, 11, was in serious condition with second- and third-degree burns over 25 percent of his body.

Prince George's County police, meanwhile, yesterday closed their investigation of the incident, calling it accidental.

Pepco officials said the youngsters apparently either touched or almost touched a 115,000-volt cable while they were walking along a steel I-beam. The I-beam carried no electricity itself, but Pepco officials said they found the burned imprints of the youngsters' sneakers on the I-beam.

The children apparently entered the building from a hill behind the concrete structure, which is set back from the road. Although warning signs are posted at various entrances to the structure, there were apparently none where the children entered.

Pepco officials said they found an eight-foot tree limb, which the children probably used to pry open a heavy metal grating to enter the structure. Police said the children may have actually broken into the building Monday night and then returned Tuesday when they were burned.

"We felt we have done all we could to ensure the buildng was safe," said Pepco spokesman David Boyce. "All reasonable precautions were in place."