Tom Mauser's passion has hardly dulled in the more than two years since his son was slain at Columbine High School. It flashed again yesterday morning as he picketed by himself outside the National Rifle Association headquarters in Fairfax.

While Mauser carried a sign that read, "My son Daniel died at Columbine -- He'd expect me to be here today," the NRA dispatched a photographer outside to snap Mauser's picture.

"Do you know what it's like to lose a child? To gunshots?" Mauser asked, as the man began to retreat. "Are you not willing to listen about my son? Have you no heart?"

The photographer, who wouldn't give his name, ducked back inside the NRA office building on Waples Mill Road. The NRA declined a request for a response to Mauser's comments yesterday.

Mauser was in Fairfax to answer charges from his visit in June, when he also picketed the NRA building and then provoked his arrest for trespassing. Trial on the misdemeanor charge was scheduled for yesterday morning, but the arresting officer, T.D. Szymanski, resigned from the police force last month and neglected to subpoena anyone to testify against Mauser. So prosecutors dropped the charge.

Mauser, 49, an employee with the Colorado Department of Transportation, has become a crusader for gun control since two teenagers walked into his son's high school in Littleton, Colo., on April 20, 1999, and fatally shot 12 students and a teacher before killing themselves.

Less than two weeks later, Mauser spoke at a news conference outside an NRA conference in Denver, then wrote a two-page letter to NRA President Charlton Heston asking such questions as why the NRA fought attempts to outlaw the type of 9mm semi-automatic pistol used to kill his son. Neither Heston nor anyone else responded to his letter or pickets, Mauser said.

After being recognized for his efforts by President Clinton in his 2000 State of the Union address, Mauser spearheaded a drive in Colorado to require background checks for buyers at gun shows. Although the legislature declined to pass such a measure, a petition drive put the proposal on a statewide ballot in November, and it passed with 70 percent approval. Mauser would like to see the background checks mandated nationwide.

Mauser said one reason for picketing yesterday was to appeal to NRA workers as they went in and out of the building at lunchtime.

"It's good for their employees to see this," he said. "They see the depth of pain, and they go back to their desk and ask, 'Why don't we respond?' "Tom Mauser, shown with his attorney Helen Leiner, returned to picket NRA headquarters in Fairfax after a trespassing charge was dropped against him.