Virginia State Police investigating the slaying of a trooper who was stabbed 42 times on the doorstep of his Manassas town house two years ago are focusing their probe on another state trooper.

Robert C. Martin, the investigator in charge of the case, said Monday that police are "99.4 percent certain" that another state trooper killed Johnny Rush Bowman before dawn Aug. 19, 1984.

Several law enforcement sources said the trooper under investigation is Perry L. Worrell, who joined the state police in 1977 and is on an unpaid leave of absence.

Worrell has not been charged, and his lawyer, Philip J. Hirschkop, said yesterday that Worrell "absolutely swears he has nothing to do with the murder of this trooper."

Worrell, 31, was injured in an apparently unrelated automobile accident early yesterday and was unavailable for comment. He was listed in serious but stable condition at Fairfax Hospital with a broken right leg and other injuries.

Bowman's death touched off one of the most extensive investigations in Virginia history, with 20 handpicked officers interviewing more than 3,000 people in 41 states. It is the only killing of a Virginia trooper that has not been solved, and police are offering a $ 28,000 reward for information leading to the arrest of a suspect.

State police and the Prince William County commonwealth's attorney called an unusual news conference in August to say that they were "99.4 percent certain" that they knew who killed Bowman. In response to inquiries by The Washington Post Monday, Martin, the special agent in charge of the state police Bureau of Criminal Investigation in the Fairfax field office, confirmed that the suspect is a member of the state police.

Police and prosecutors say they have not sought a grand jury indictment because they do not have sufficient evidence to get a conviction.

Investigators zeroed in on a state trooper as the primary suspect because of "the totality of the circumstances," Martin said.

He added: "I don't care whether he's a member of this department or any other department. Murder is murder . . . . I'm not concerned about embarrassment to the department. We've always had a history of cleaning our own house."

Police have speculated that Bowman was killed as a result of "some personal problems" involving the assailant, the dead trooper and his wife Terri.

Police said Bowman, 31, was slain when he opened his front door to admit the assailant.

In August, state police said they had a suspect under constant surveillance. They would not say whether the suspect is still under surveillance.

Hirschkop, Worrell's lawyer, said in an interview that Worrell had been followed by police, "I guess in an effort to scare someone."

In an interview Monday, Worrell's wife Melanie referred to "what we've been through," without elaborating.

Asked if the Worrell residence has been watched, she said that for the past two years, someone telephones about twice a day and hangs up when the telephone is answered.

Martin acknowledged yesterday that a trooper was a suspect almost from the beginning of the investigation but said that after a few months the lead seemed to reach a dead end.

At the news conference in August, police said that information gathered in the previous two months had produced "dramatic evidence."

That evidence, Martin said in an interview Monday, pointed to a trooper as a suspect.

He added: "We simply don't have the evidence to, in our judgment, sustain a conviction."

Martin, a 30-year veteran of the force, has said he is "certain that [the] homicide will be solved in view of the fact that more than one person knows who killed Bowman."

State police Superintendent Col. R.L. Suthard said in a telephone interview Monday, "I don't know of any person who is a member of the Virginia State Police who is a primary suspect in that case." After he confirmed that Worrell is on an unpaid leave of absence, however, he changed his statement to, "I don't know of any person who's on the payroll of the state police who is currently a prime suspect."

Law enforcement sources said Worrell was placed on unpaid leave within weeks of the slaying. A spokesman for the state police in Richmond would not give the date of the action or the reason for it.

Hirschkop said Worrell was placed on leave for medical reasons because he was upset about the death of Bowman.

The Bowmans and the Worrells had been "friends for a very, very long time," Hirschkop said yesterday.

Worrell, who lives in Dumfries and works for a Prince William County towing and wrecking ser- vice, was on call about 2 a.m. yesterday when he was injured in an accident.

According to state police, a trooper noticed a disabled car on the shoulder of an exit ramp off I-95 at Dale City.

When the trooper radioed for a wrecker to tow the vehicle, Worrell responded. While Worrell was bent over the engine, a third vehicle came down the ramp and smashed into the rear of the disabled car.

The impact pinned Worrell between the disabled car and his wrecker, police said.

The owner of the disabled car also was hurt, although the extent of her injuries could not be determined yesterday.

Police said the driver of the third vehicle, whose name was not released, apparently had fallen asleep at the wheel.