A fight in a King Soopers parking lot in July has led to the arrest of a man who prosecutors believe is a violent sex offender responsible for three unsolved rapes in the Denver area, including an incident involving a 13-year-old girl.

William Burton Costello, 47, allegedly shoved a 69-year-old anti-abortion petitioner from his bike on July 6, breaking the man's hip.

His arrest for Class 3 felony assault on an at-risk adult allowed police to gather Costello's DNA and run it through the federal CODIS database of DNA.

Colorado law changed in 2010 with the enactment of Katie's Law, which allows authorities to collect DNA from people charged with felonies who have not yet been convicted.

On Oct. 30, Costello's DNA came back a match for three sex assaults dating back as far as 2008, two of which involved physical force and included Costello telling alleged victims he was a police officer, according to arrest affidavits.

The Denver District Attorney's Office on Friday charged Costello with two counts of second-degree kidnapping, two counts of sexual assault on a child, three counts of sexual assault and two counts of impersonating a peace officer.

In the first incident, Costello allegedly picked up a 13-year-old runaway on March 22, 2008 and drove her to an alley, where he sexually assaulted her, police say. He told her his name was "Will" and he had a 2-year-old boy in the car at the time, according to affidavits.

When he dropped off the teenager at a 7-Eleven, he allegedly flashed a handgun before letting her go.

More than two years later on Aug. 12, 2010, a man driving a black Mercedes identifying himself as "Will" offered to drive a 22-year-old woman through a Del Taco drive-thru, since the dining room was closed.

Instead, police say Costello drove her away from the restaurant, told her he was a "crooked" cop and sexually assaulted her.

And finally, police believe Costello identified himself as a police officer and ordered a 49-year-old woman who was waiting at a bus stop into his Mercedes on Sept. 28, 2011.

"The suspect told her to relax, that they were just going to have a few drinks," according to the affidavit. He later told the woman, "He was a detective and she was either going to have sex with him or provide him with oral sex, or she was going to be taken to jail."

When the woman told Costello to arrest her, he became physically aggressive and held her down, the affidavit says. The woman was eventually able to escape his car.

The two adult women described their assailant similarly.

Costello was ultimately caught following the altercation in the King Soopers parking lot because a witness at the scene took down his license plate number.

Jessica Fender: 303-954-1244 , jfender@denverpost.com or twitter.com/oh_fender