Keith J. Gardner, who has been sought by Fairfax County police since his parents and his grandfather were found stabbed to death in an old fallout shelter at their home in the Lorton area, was arrested last night in Pensacola, Fla., authorities said.

Gardner, 39, was identified by Fairfax police last week as a suspect in last month's slayings, but he was arrested on a fugitive warrant charging him only with a probation violation stemming from a 1989 drug conviction.

A half-dozen officers from the Escambia County sheriff's department and the local FBI office made the arrest just after 8 p.m. Central time at a hotel near Highway 29, a major north-south route through Pensacola, officials said.

Ricky Shelby, an investigator with the Escambia County sheriff's department who participated in the arrest, said Fairfax County police received a tip that Gardner was in the Pensacola area. He said a makeshift task force was assembled and set up surveillance outside the Landmark Inn and arrested Gardner when he emerged from his room last night.

He said Gardner was taken into custody without incident and taken to the Escambia County jail.

"He offered no resistance; he understood we had a job to do," Shelby said. "When he left the motel property [shortly before being arrested], he noticed one of the surveillance vehicles. He turned and started walking the other way. Then he saw another surveillance vehicle. When he saw the second surveillance vehicle, he approached them and was arrested without incident."

Police had been trying to locate Gardner since May 11, when the three bodies were discovered in the below-ground, Cold War-era shelter behind the Gardner family home in the 9200 block of Gilmore Drive. Police had offered a $5,000 reward for information leading to his arrest.

Two neighbors discovered the bodies of Jimmy Gardner, 64, his wife, Jannis, 63, and his father, Elmer, 90.

Neighbors and friends said they became concerned when they noticed the three elder Gardners hadn't been seen in a week. Keith Gardner, who had moved back into the home two weeks earlier with his 19-year-old wife, told at least two friends of the family that the other family members had gone to Arkansas, where Jimmy Gardner owned rental property.

Police said that the bodies had been covered in limestone powder and that the entrance to the shelter was caulked shut from the outside. Police also found blood in the victims' bedrooms, and investigators found four knives wrapped in towels in Keith Gardner's bedroom.

The night after the bodies were discovered, Fairfax County police found Gardner's Chevrolet S-10 pickup truck in the parking lot of a restaurant that was within walking distance of a hotel, a motel and the Springfield-Franconia Metro station.

Keith Gardner has a Virginia criminal record that dates back to 1986 and includes burglary convictions in Fairfax, Prince William and Spotsylvania counties, officials said. The alleged probation violation stems from a 1989 conviction in Fairfax on drug distribution and weapons counts; he was sentenced to eight years in prison, though it was unclear how much of that sentence he served before being released on probation.

Gardner was permitted to serve that probation in Florida beginning in March 1998. Six months later, he was convicted there of two misdemeanor charges for loitering outside a convenience store and fleeing police to avoid arrest, officials said.

CAPTION: Keith J. Gardner is a suspect in the slayings of his mother, father and grandfather at their Lorton home.