Virginia state police investigators and prosecutors said yesterday they are "99.4 percent certain" they know who killed an off-duty state trooper at the front doorstep of his Manassas town house two years ago today, and expect to make an arrest soon.

The officials called a news conference to say that information gathered in the last two months has produced "dramatic progress" in the investigation of the slaying of Trooper Johnny Rush Bowman.

Bowman, 31, was stabbed 42 times when he answered his doorbell before dawn Aug. 19, 1984.

Police said they have the suspect, whom they identified only as white, male and a Virginia resident, under constant surveillance.

Paul B. Ebert, the Prince William County prosecutor, said there had been no arrest so far because "we haven't had enough evidence to convict." Ebert would not say what further evidence would clinch the prosecution's case.

The slaying touched off one of the most intensive and costly investigations in the history of the Virginia State Police. It is the only killing of a Virginia trooper that has never been resolved.

The officials said there was "a high degree of familiarity" and "an emotional involvement" between the suspect and Bowman and Bowman's wife Terri.

In a statement, the officials said Bowman, a five-year member of the police force, was killed because of "personal problems" between the trooper, his wife and the suspect. The statement also said that the suspect had probably been at Bowman's house before the slaying. The officials said the relationship was not a love triangle.

Terri Bowman could not be reached for comment yesterday.

"I'm confident we know what happened that night," said Robert C. Martin, the chief state police investigator in charge of the Bowman case. "I think it's just a matter of time" before an arrest is made.

Martin added that he is "99.4 percent certain" he knows the identity of the killer. Ralph Marshall, another state police investigator, said he is "almost 100 percent convinced, certainly beyond any reasonable doubt."

Martin, citing an FBI profile of the suspect as well as information gathered by police, said the suspect had become increasingly depressed and anxious since the murder, and "cannot cope with his crime."

He said that while the suspect probably regrets his acts, he may be increasingly prone to violence and could turn upon an associate whom he thinks has been disloyal.

Martin said he was certain that "more than one person knows who killed Bowman," but would not elaborate.

"These people who know what happened," said Ebert, "they certainly will be treated fairly and justly" by the authorities if they come forward.

Police originally reported on the basis of information obtained at the time that the trooper opened the door to his home at 4:15 a.m. after the killer used an obscenity and said, "I'm from the power company."

Ebert said yesterday he was "not convinced" the assailant identified himself as from the power company.

Police describe the attack on Bowman as one of the most vicious crimes they have encountered. The trooper was stabbed with what police believe is a folding knife with a 4-inch blade. Police said the attack lasted about three minutes, and that the attacker was in a frenzy, stabbing the trooper even after he had fallen to the ground. The killer then ran away. The knife has not been recovered.

Investigators have said repeatedly that the case would not be closed until there is an arrest.

In the months after the murder, nearly 20 hand-picked special state police agents were assigned full time to the Bowman case, working 12-hour shifts daily. They interviewed more than 3,000 people in 41 states, checked thousands of leads and amassed a small library of information about Bowman, his wife, their friends, acquaintances and distant contacts.

Police are offering a $28,000 reward for information leading to the arrest of a suspect. That is one of the largest rewards in a murder case in the state's history.