For a few days, it was the best lead in years to Montgomery County's longest and most maddening police investigation -- the unsolved 1975 disappearance of the two Lyon sisters.

On March 19, detectives announced that Fred Howard Coffey Jr., a 41-year-old convicted child molester and suspected killer facing trial in North Carolina, was known to have been in Montgomery County just a week after the Lyon girls vanished while walking home from a shopping center.

Police appealed to the public to help establish Coffey's movements, to see if he had arrived in Montgomery by March 25, 1975. On that day, 12-year-old Sheila Lyon and her 10-year-old sister Katherine went to Wheaton Plaza, had lunch and began walking back to their Kensington home.

The girls, daughters of prominent radio disc jockey John Lyon of WMAL, were never seen again.

Today, Coffey's presence in Montgomery continues to tease police, but they have been unable to link him to the Lyon case or to the 1975 sexual assault and slaying of 15-year-old Kathy Beatty of Aspen Hill, which occurred less than a mile from where Coffey once worked.

Despite an estimated 500 calls from the public in the days following the March news conference, the disappearance and slaying remain unsolved.

"We have pretty much run the string out," said Montgomery police Sgt. Gary Smith. "We're disappointed, but we're not really surprised."

Over the years, the search for the Lyon girls has included searches by bloodhounds of wooded areas, the use of a psychic and repeated false sightings. In 1982, police fruitlessly dug up a yard in Suitland where a convicted killer had lived.

Until March, none of the thousands of hours of county police investigation had produced Coffey's name.

His name turned up in Bristol, Va., where Coffey later resided and where he became a suspect in the August 1986 slaying of an 8-year-old. Bristol police informed Montgomery detectives that Coffey had worked at Vitro Corp., a defense firm in Aspen Hill, as a computer data system employe from April 24, 1975, to July 31, 1975. According to company records, Coffey was in Montgomery on April 1 of that year to interview for the job.

In Charlotte, N.C., Coffey awaits trial on a first-degree murder charge in the 1979 slaying of 10-year-old Amanda Ray. He also has been sentenced to 50 years in prison after pleading guilty in Caldwell County, N.C., to molesting three children.

Smith said Coffey's lawyers have refused to let Montgomery detectives interview him, and investigators have never learned whether Coffey was in Montgomery during the crucial week before he was interviewed at Vitro.

Police tried to pinpoint when Coffey first came to a Gaithersburg motel where he was known to have stayed. They also tried to get old motor vehicle records to verify reports that he had bought a car in Montgomery about the time of the disappearance. Smith said police had hoped to link Coffey to a set of keys found near the beaten body of Kathy Beatty.

In both cases, the records had been destroyed, Smith said.

"There has been a lot of good effort, but the results have been disappointing," Smith said. "You put yourself in the place of Mr. and Mrs. Lyon, and it is frustrating to get a glimmer of hope and then have it fade away."