William Costello charged in three unsolved rape cases from 2008, 2010, 2011

Costello also charged with impersonating officer

DENVER - DNA collected four months ago has helped police stop a suspected serial rapist.

William Costello , 47, was arrested in July and charged with assault on an at-risk adult following a confrontation between Costello and a man collecting petition signatures, according to the Denver District Attorney's office.

Prosecutors said Costello’s DNA was taken at the time of his arrest pursuant to Katie’s Law. That DNA was then linked to three unsolved sexual assault cases occurring in 2008, 2010, and 2011.

Prosecutors now allege that Costello sexually assaulted a 13-year-old girl in March 2008, a 22-year-old woman in August 2010, and a 49-year-old woman in September 2011. 

Costello is being charged with two counts of kidnapping,  two counts of sexual assault on a child, three counts of sexual assault and two counts of impersonating a peace officer, the D.A.'s office said.

The charges further allege that in the latter attacks Costello claimed he was a police officer, prosecutors said.

Arrest affidavits state that in the first incident, Costello picked up a 13-year-old runaway in 2008 and sexually assaulted her with a 2-year-old boy in back seat of the car.

Two years later, in 2010, the affidavit states that a man driving a black Mercedes offered to drive a 22-year-old woman through a Del Taco drive-thru because the dining room was closed, but instead he drove her away.

The report states that he claimed to be a "crooked cop" and sexually assaulted her.

In the third incident, on Sept. 28, 2011, the affidavit states that Costello claimed to be a detective and ordered the 49-year-old woman into his Mercedes.

"The suspect told her to relax, that they were just going to have a few drinks," the affidavit states.

The woman was eventually able to escaped the car, but not before being sexually assaulted.

In July, Costello was accused of shoving a 69-year-old anti-abortion petitioner from his bike, breaking the man's hip.  It was that felony charge that lead to Costello's DNA being taken.