Fairfax County police decided to use tear gas in an attempt to force a Burke man from his home after a 4 1/2-hour siege Monday when they became convinced they would never be able to persuade him to walk out on his own, a senior police official said yesterday.

The man, 50-year-old Ralph J. Carrigan, a father of four and a Federal Trade Commission lawyer, shot himself to death with a handgun moments after the tear gas was hurled into his home, police said.

"We were dealing with someone with whom we couldn't establish any communication," said Maj. Alan L. Barbee, the officer who directed the police operation Monday. "The key factor was there was no communication, no willingness to communicate on his part."

The siege at the Carrigan home at 5201 Sterne Ct. was triggered at about 11:30 Monday morning when Carrigan fired two shots at his 17-year-old daughter Ellen, police said. The girl, who was sleeping in her room at the time, was wounded in one shoulder but managed to leap through a bedroom window and flee. No one else in the family was home at the time, according to police, who declined to discuss what prompted the shooting.

Carrigan had been arrested Sunday night and charged with simple assault, a misdemeanor, against the 17-year-old daughter, police said. He was released after a magistrate's hearing.

Yesterday, all was quiet on the dead-end street where the Carrigan home is nestled amid trees. Family members, who declined to discuss the shooting, were being sheltered by neighbors since the front door of their home had been smashed in by police trying to force entry Monday.

According to a family friend, Sue Rowell, Ellen Carrigan was "doing well" in Fairfax Hospital where she was able to receive visitors yesterday. While Rowell declined to discuss the shooting in detail, she noted that Carrigan, a 21-year employe of the FTC, "was maybe not well, not himself for the last year or so . . . There was unhappiness, though no one could put his finger on what was causing it. He had stomach problems, he was ill a year ago . . . There's an element of shock surrounding the whole thing. It was really a beautiful family, very strong."

Police said Carrigan apparently barricaded himself inside the house after his daughter was shot. Not until three hours later were police able to determine for sure that he was still inside because he had refused to answer the telephone or respond to pleas over a bullhorn to come out. Only after police forced their way into the home about 3 p.m. were they able to elicit a response from Carrigan, Barbee said yesterday.

"They heard some noise from another room" -- it was the kitchen -- Barbee said. But when they called out to Carrigan, his only response was that they should "get out of my house," Barbee said.

For the next hour and 20 minutes, Barbee said, police tried without success to get Carrigan to speak with them. Finally, because they believed he was unlikely ever to communicate with them, Barbee said, the decision was made to attempt to force him out.

"In a lot of situations like this, there is an early state of volatility and then the person involved . . . will reach a stage where we can communicate with him," Barbee explained. "You can talk them down. Unfortunately, we couldn't do that in this case."

Barbee said the decision to use the tear gas at 4:20 was made for several reasons. One, he said, was that darkness was approaching, and any attempt to flush out an individual in the dark "was not to our advantage." In addition, he said, police had to consider whether "we just wait and block off the neighborhood and the streets and keep people out of their houses or do we attempt to do something?"

Complicating the situation, he said, was the fact that the man's family could not be asked to assist in efforts to persuade Carrigan to give up because "it probably would have made the situation worse." There were bad feelings among family members, one source familiar with the case said yesterday.