A sex crime investigation that began last year in rural Southwest Virginia led Montgomery County police yesterday to announce their "most promising lead" in the county's longest-running police investigation -- the unsolved 1975 disappearance of the Lyon sisters.

The Lyons, daughters of prominent radio disc jockey John Lyon of WMAL, were last seen March 25, 1975, after walking from their Kensington home to the Wheaton Plaza shopping center to look at Easter exhibits and have lunch. They never returned.

Police said yesterday that the disappearance of 12-year-old Sheila Lyon and her 10-year-old sister Katherine may be linked to a Virginia man, now in custody in North Carolina, who is suspected in sexual molestations and slayings of children in at least three states.

At a Rockville news conference, police said the man, Fred Howard Coffey Jr., 41, who has been convicted of molesting three children in North Carolina, was in Montgomery County near Wheaton Plaza on April 1, 1975, seven days after the Lyon girls vanished.

Police also said the last person seen with the Lyon girls was a man at Wheaton Plaza who had a briefcase containing a tape recorder. In other cases in which Coffey is a prime suspect, victims have been lured by a man using devices including a fishing pole and a metal detector, police said.

"That was a similarity that struck me, the use of one of these gimmicks," said Montgomery Detective Bill Campbell. He said the man with a recorder was described by youngsters at Wheaton Plaza as "being very much older" than Coffey, although he added, "children gauging the age of adults can be off by a number of years." Campbell said Coffey's hair was prematurely gray, but that it probably would have been dark at the time.

Police said Coffey also may be linked to a second Montgomery case -- the 1975 slaying of 15-year-old Kathy Beatty of Aspen Hill, who lingered unconscious for a month after being sexually assaulted and beaten.

The search for the Lyon sisters has consumed the attention of area residents for more than a decade and produced thousands of fruitless leads.

Despite searches of wooded areas by bloodhounds, the use of a psychic and countless false sightings of potential suspects, no trace of the girls was ever found.

None of the thousands of hours of county police investigation produced Coffey's name, police said. Authorities learned of his existence in December, after Coffey became a suspect in an August 1986 slaying of an 8-year-old boy in Bristol, Va.

Police said they don't know where Coffey was on the day of the Lyons' disappearance, and they appealed to the public for help.

"His activities and whereabouts . . . are not at all clear. He is very likely to have been in and out of this area in search of employment," said Campbell, who urged anyone knowing of Coffey's whereabouts at the time to call Montgomery detectives.

The FBI and Bristol authorities were investigating the August 1986 suffocation death of Travis Shane King when they informed Montgomery investigators that Coffey was in Maryland in early 1975. Coffey has not been charged in the Bristol case, police said.

After the tip from Bristol, Montgomery investigators learned that Coffey worked at Vitro Corp. in Aspen Hill as a computer data system employe from April 24, 1975, to July 31, 1975. According to company records, he interviewed for a job there on April 1, Campbell said.

When he quit in July, he told his employers that his wife and daughter had been injured in a Kentucky car accident. Campbell said yesterday that the story was false, and was identical to one he used in leaving another job.

In Charlotte, N.C., Coffey has been charged with first-degree murder in the 1979 slaying of 10-year-old Amanda Ray, police said. He also is considered a suspect in the death of a 12-year-old girl in Charlotte.

In addition, he has pleaded guilty in Caldwell County, N.C., to molesting two girls and a boy, ranging in age from 9 to 11, and was sentenced to 50 years in prison. He is expected to serve at least 15 to 20 years before becoming eligible for parole, said Caldwell County prosecutor Robert Thomas. Coffey is now in a county jail in Charlotte awaiting his murder trial there. Staff writer Joseph E. Bouchard contributed to this story.