Trying to look inconspicuous, Det. Sgt. Robert Kiesel sat in his unmarked police car with his finger on the trigger - of a video camera. He was on stakeout duty, waiting to spot the target of his investigation: a Nassau police lieutenant.

Kiesel had a job few officers wanted, one he sometimes didn't want himself. It had brought him bomb threats, slashed tires, an ulcer and high blood pressure.

As a detective sergeant in Nassau's police medical unit, Kiesel investigated officers suspected of malingering. The good part of the job was knowing he'd saved the taxpayers some money. One of the bad parts was feeling like an outcast in the department.

It was April, 1985, and Kiesel peered through the rain-spattered windshield at the travel agency owned by the lieutenant's wife on Middle Country Road in Middle Island. Eventually, Lt. Mauro Fiore drove up and strode briskly into the agency. Kiesel taped him, watching closely for a limp. He saw no sign of one, he said in a recent interview.

Fiore, then 46, a desk officer, had tripped over an electrical box in his station house in November, 1984, and had had a knee operation, according to department records. Now he was on sick leave. Sometimes modern surgical techniques could make bad knees as good as new. Kiesel was in the parking lot across from Everyday Travel to answer one question: Did Fiore still have any serious knee problem?

For several days Kiesel said he watched the agency as Fiore was making repairs to the office. Kiesel taped him walking back and forth to his car for supplies and tools, carrying lumber and a box of ceiling tiles, and putting out the garbage. Through the big front window, Kiesel said, he could see Fiore climbing up and down a ladder. Throughout all of these jobs, Fiore had the gait of a young man. He never limped.

Then the department called Fiore in for questioning, and Kiesel tried one of his favorite tricks. Sometimes this trick gave his videotapes a comic ending. After taping an officer who didn't know he was being watched, Kiesel would also tape him as he came up the department's front steps for questioning. The idea was to show the contrast in how he walked when he thought someone might be watching.

When Fiore walked up the steps of police headquarters, Kiesel saw through the viewfinder what he'd been expecting. Fiore looked like a different person from the one who'd been working so efficiently on the travel agency renovations. As he climbed the police department steps, he limped like a movie pirate with a peg leg.

Kiesel's investigation resulted in the department's filing seven disciplinary charges against Fiore for violating sick-leave rules. Fiore pleaded guilty. He was fined 35 days' pay and suspended for 30 days without pay.

Fiore returned to work, but continued to complain of problems with his knee, and filed for a disability pension. He then reported another fall while on duty. John Sharp, then commander of the medical unit, said he offered to send state officials the videotapes. As in other cases, they didn't respond to the offer, Sharp said.

Nevertheless, the state twice turned down Fiore's application for a disability pension based on doctors' findings that although he had a disability, he could still perform his desk officer duties. He retired on a regular pension in 1987, but continued to fight for the disability pension.

Nassau police unions argued successfully on behalf of Fiore and others who had been similarly rejected that disability applicants should be judged against the full duties of a police officer. In 1990, the state reconsidered, and Fiore, then 52, and several others got their disability pensions. He collects $62,974 a year, tax-free.

Asked for comment, Fiore denied that Kiesel had caught him doing any renovation work at the travel agency. "I went down to my wife's office to sit around and drink coffee with her ....I wasn't working there. I was just basically going down there and hanging out."

For Kiesel, the other bad part of the medical job was the sense of futility. Cops he didn't respect, cops he considered an embarrassment to the job, were beating the system, as he saw it, and all he could do was give them some temporary problems.

In the long run, when the big money was at stake, they came out the winners. "That's what gets you upset, when you see abuses go through," Kiesel said. "We've sent Albany letters, reports, videotapes on cases where we knew the guy was faking. Never got a response back. Not even, 'I got the tape, thank you.' They should be taking a much harder look at these cases."