A federal jury yesterday found that a Fairfax police investigator violated a Falls Church man's civil rights when he arrested him for allegedly sexually abusing and illicitly photographing his two children, charges that were later dropped.

The jury awarded William J. Kelly Jr., a Falls Church photographer, $55,000 in damages. Kelly was imprisoned for seven days last year after being arrested on rape and indecent liberties allegations.

"They are probably the most vicious accusations made against any father anywhere," Kelly's attorney, George L. Freeman Jr. told the U.S. District Court jury in Alexandria. Kelly "was in jail for seven days . . . . He just couldn't believe what was happening to him."

After the prosecutor dropped all charges against him, Kelly sued William H. Whilden and the Fairfax County Police Department for allegedly "coercing" his children into making statements that he sexually abused them. The 45-year-old photographer contended that when Whilden asked the children if they ever "fooled around" with their father, they said "no."

Only after Whilden warned the children that they might be sent to a juvenile detention home if they didn't tell the truth, did they change their story, Freeman argued.

Kelly's 10-year-old daughter backed up her father's account in court.

The girl testified at the opening of the three-day trial that she initially told Whilden that her father had not indecently touched her or photographed her.

But she continued: "Then he {Whilden} said if you don't tell the truth, you'll go to juvenile jail . . . so I told him yes . . . . I thought if he heard what he wanted to hear everything would be all right."

Both the girl and her 12-year-old brother also testified that it was they, and not their father, who took the four nude photographs that launched Whilden's investigation.

The children, who live with their mother and stepfather in Florida, said they snapped nude photographs of each other and then inadvertently gave the roll of film to their father to have developed.

A Colorfax laboratory technician in Silver Spring, who was developing the film for Kelly, saw the nude photographs on the roll and called police.

Assistant Fairfax County Attorney Robert M. Ross defended Whilden, a 14-year member of the police department who is assigned to the child services unit, saying he did a "professional job."

Whilden's attorneys argued that the photographs were not of the type that children snapped of each other. They denied that Whilden overstepped the bounds of his authority and coerced the children into giving him the story "he wanted to hear."

Kelly's lawsuit also charged that Whilden and the Fairfax County Police Department had maliciously prosecuted and maliciously imprisoned him, charges that the jury found were not substantiated.

Kelly, who was arrested June 13, 1986, was initially held without bond.

Seven days later, when the bond was set at $20,000, he was released from Fairfax County Detention Center. He had sought $5 million in damages.

"I had hoped that the verdict would have been for a larger amount," Freeman said. "But we're satisfied."