How do investigators catch a serial killer?

In May of 1992 Kenneth McDuff was taken into custody and brought to McLennan County, TX, ending a months-long manhunt for the serial killer.

"He was soulless. People say, 'Oh everyone has a soul.' Not this guy," says McLennan County Sheriff Parnell McNamara.

He was a U.S. Marshal during the search for McDuff and played a key role in his arrest.

"Kenneth McDuff in my opinion, and everyone else's, was the worst of the worse. He was the most evil killing machine that I ever encountered."

McDuff's killing spree started at an early age. He was convicted in 1966 of killing three teenagers near Fort Worth. He was almost put to death twice before his sentence was commuted to life.

Then, due to prison overcrowding, he was released in 1989. That's when young women began disappearing from all over Central Texas, including a woman named Colleen Reed from Austin. Eventually investigators linked one of those disappearances to McDuff after his car was found abandoned near the Waco convenience store where a missing girl worked.

A team of U.S. Marshals from all over the country was assembled in McLennan County to search for McDuff.

"You're racing against time," McNamara said. "You know this guy is out there wanting to kill. You just hope you can get there before he kills again."

McDuff was featured on American's Most Wanted and within a couple days, a tip came in that he was in Kansas City. McNamara and a team of U.S. Marshals flew there to arrest him and bring him back to Waco.

"The feeling we all had was such relief because we were frantically trying to catch him and stop him from killing because we knew that as we were pursuing him, this guy was killing people all along," McNamara said.

Investigators believe McDuff likely killed between 15 and 20 people. How someone can become a serial killer is something researchers are still working to determine.

Forensic psychologist Dr. Alissa Sherry says a killer's past can provide some answers.

"(Factors like) severe childhood abuse, physical sexual, emotional abuses, severe neglect," Sherry said. "Things like patterns of being humiliated, humiliated by their parents, humiliated by peers in school."

When investigators have a serial killer case on their hands, Dr. Sherry says they're dealing with a psychopath.

"A psychopath is someone who typically lacks empathy, engages in behavior for their own personal gain and reward, has a difficult time connecting with people emotionally."

That's exactly the kind of person McDuff was, according to McNamara, right up until the day he was executed in 1998.

"He never apologized, never apologized to the families for what he did," McNamara said. "Cold blooded to the end."