A dozen years after the investigation opened, the missing persons case file on Katherine and Sheila Lyon has grown to fill three boxes the size of orange crates at Montgomery County's Wheaton police station.

They sit in a storage area, lately untouched.

"You get to a point where there's not much left to pursue," said Sgt. Gary Smith, a supervisor of the detective squad in charge of what has become Montgomery's most noted unsolved case. The girls, daughters of WMAL radio disc jockey John Lyon, were walking home to Kensington from Wheaton Plaza when they disappeared on March 25, 1975.

Sheila was 12, Katherine, 10.

"Leads and leads have been checked," said Smith. "They've been checked and double-checked and rechecked." Only one still shows any promise, he said. Fred Howard Coffey Jr., a convicted child molester, who is also suspected in a slaying, is known to have been in Montgomery at about the time of the disappearance. He is awaiting trial in Charlotte, N.C., in the slaying of a 10-year-old girl. Montgomery detectives have been eager to speak with him for some time.

Coffey, though, has chosen to remain silent.

"As his case moves along down there, we're hoping something happens to change his mind," said Smith. Thus far, he said, detectives here have been unable to link Coffey to the Lyon girls.

"We're in a holding pattern," he said.