After he found his teen daughter dead, Roger Kemp hunted down her murderer – and now helps find other fugitives

The memory haunts Roger Kemp to this day.

One evening in June 2002, after a brief search for his missing 19-year-old daughter Ali, Kemp, now 67, found her strangled and left under a tarp at the local pool where she worked.

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“Ali was our precious first born,” Kemp, of Leawood, Kans., recently told PEOPLE, as he remembered weeping over his daughter’s battered, lifeless body.

“She loved soccer and poetry. She was voted nicest girl in the eighth grade. In high school she was so involved, they made an exception to the number of times your picture could show up in the yearbook,” he says.

Heartbroken, Kemp vowed to bring her killer to justice with a novel idea. Armed with a police composite sketch of a white male suspect, Kemp went to Lamar Advertising, a local firm, to put up four “wanted” billboards in town.

Amazingly the tactic worked: Tips poured in, leading to the 2004 arrest of serial offender Benjamin Appleby, then 29, now serving 50 years.

Inspired by Kemp’s idea, soon other police departments around the country adopted similar billboards. To date, in Kansas alone, they have already helped authorities capture more than 30 fugitives.

Despite the pain he’ll always carry, for Kemp, who received a Presidential Citizens Medal for his work from President Obama last fall, there’s a solace in knowing he’s helping police take other killers off the streets.

“The trauma of the day I found her will haunt me the rest of my life. The moment Ali was taken from us, it was like the light went out in the world,” says Kemp, whose Ali Kemp Educational Foundation, founded in memory of Ali, has taught more than 46,000 students in basic self-defense.

“For weeks I had to fight just to get out of bed. But I had to keep going, not just for Ali but for our family,” says Kemp. “When she was with us, Ali made a difference in people’s lives. Through this work, she still is.”