A Marine lance corporal, once a guard at Camp David, who was arrested Wednesday in the slaying of legal secretary Shannon Anne McMillan of Silver Spring was charged after Montgomery County police combined legwork and science to narrow the field of suspects, detectives said yesterday.

The suspect, Arnold Jason Williams, 21, described as an exemplary Marine with no criminal past, lived on the same floor as McMillan at the Hampton Point North apartment complex until Oct. 3, two days before she was found sexually assaulted and stabbed to death in her living room, investigators said. One detective said Williams, who is married, formed a close relationship with McMillan, 22, after she moved to the complex in September.

The detective, who asked not to be identified, said recent laboratory analyses of a fingerprint left in the apartment and semen found in McMillan's body pointed to Williams as a strong suspect in a killing that had baffled investigators for weeks. He said police used the forensic evidence Wednesday to obtain a warrant to search a borrowed car Williams had been using and found a knife they believe was the slaying weapon.

The detective would not discuss a motive for the slaying. He said McMillan was stabbed several times and was dead when her assailant sexually assaulted her and slashed her throat.

McMillan, described by friends as an attractive but troubled young woman from a broken home, drifted into and out of relationships with several men after moving to the Washington area from Texas four years ago. Investigators said she met Williams on Sept. 13, the Sunday she moved to Hampton Point North from a condominium she had shared with a boyfriend in Annandale.

Williams, who was among an estimated 100 people questioned by a team of detectives in the days and nights after the killing, told police he helped McMillan carry a mattress into her apartment on Sept. 14 and never crossed her doorstep again, according to a detective.

But the FBI performed laboratory tests on newspaper pages found near McMillan's partly clothed body and recently discovered Williams' left thumb print on one of them, the detective said. He said the page, from the Washington Post sports section, was dated Sept. 25.

"That was the crucial turning point," the detective said. "We had an individual who said he'd only been in there once, then we find a fingerprint from days later. That's when we keyed on him."

The fingerprint was among hundreds lifted in the apartment and compared with fingerprints taken from many of those interviewed by police.

While the FBI was testing the newspaper pages, the Maryland State Police crime laboratory was analyzing the semen samples.

Without discussing the McMillan case specifically, an specialist at the laboratory said that by examining a person's semen, a serologist usually is able to identify several chemical characteristics of that person's blood. With data gathered from blood banks across the country, the serologist can then estimate what percentage of the population is likely to have blood with that combination of characteristics.

Only a "very small percentage of the population" has the same blood characteristics as the man who raped McMillan, according to one detective. He said Williams is one of them, based on laboratory tests of a blood sample Williams agreed to give police.

The detective said semen analysis was used early in the investigation to eliminate at least one of McMillan's former boyfriends as a suspect.

With the forensic evidence, the detective said, investigators obtained warrants to search Williams' apartment on Hampton Point Drive and a Nissan 200-SX that Williams borrowed from a friend several weeks ago. The apartment is a short distance from Parkford Manor Terrace, where Williams and his wife had lived in the same building as McMillan.

The detective said McMillan's car keys and purse, reported missing after her death, were not found.

Williams, from Camden, N.J., entered the military in June 1985 and became one of a select group of Marines chosen to guard Camp David, the presidential retreat in Maryland's Catoctin Mountains, according to Sgt. C.D. Chambers, a spokesman at the Marine Barracks at Eighth and I streets in Southeast Washington.

The Marine Corps considers "8th & I" its showcase barracks. The Marine Band, the Silent Drill Platoon and other elite ceremonial units are based there, and the barracks provides the guard detachment for Camp David.

Chambers said Williams was assigned to 8th & I in late 1985, was detached to Camp David from June 1986 to last March, then returned to the barracks to work in the "press shop," ironing uniforms.

He said Williams' eight-month rotation to Camp David was normal, and that his service record was exemplary.

The Marine Corps chooses its future Camp David guards from recruits still in basic training, Chambers said. He said officials evaluate hundreds of recruits each year, and select fewer than 1 percent of them for eventual assignment to the 8th & I barracks.