DENVER - A well-known Denver real estate broker accused in a series of cold-case sexual assaults in Denver has been charged in a fourth case.

On Friday, Denver District Attorney Mitch Morrissey formally charged William Costello, 47, with second-degree kidnapping and two counts of sexual assault in connection with a crime that took place in 2011.

The new charges allege that on July 30, 2011, Costello assaulted a 31-year-old woman in the area of 16th Avenue and Steele Street in Denver.  The two were acquainted with each other through a mutual friend, Morrissey said.

Costello was first arrested and charged in July 2012 with assault on an at-risk adult following a confrontation between Costello and an anti-abortion activist who was collecting petition signatures.  

Costello’s DNA was taken at the time of his arrest pursuant to Katie’s Law, which requires anyone charged with a felony to have their DNA put into a database.  

The DNA was then linked to three unsolved sexual assault cases occurring in 2008, 2010, and 2011.

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

The 2010 incident, 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.  The case was open until a DNA match came back, linking Costello to the crime.

On Sept. 28, 2011, an arrest affidavit states that Costello claimed to be a detective and ordered a 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 stated.

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

Costello posted a $100,000 bond in the latest case and has been released.  He's due in court on March 14 for formal advisement.