Kooyman's defense attorney, Rudy Bautista, said prosecutors dropped the rape case against his client when they found "glaring inconsistencies" in the alleged victim's story.
"These inconsistencies really hurt her veracity, and other aspects of her lifestyle raised serious questions about the case," Bautista said. "The prosecution felt they didn't have enough evidence to pursue the case to trial."
According to charging documents, Kooyman met a woman at a karaoke bar on June 15, exchanged phone numbers and took her out the next day to go swimming and drinking.
Somewhere along the way, the woman began to feel groggy and asked Kooyman to take her home, police said. Instead, he took her to his home in Sandy, where he showed her a case of handguns before she passed out, the documents allege.
Bautista acknowledged that police found a gun in Kooyman's home, but noted it was in his mother's bedroom and does not belong to his client.
"To say he possessed a firearm would be a stretch," Bautista said.
The woman alleged Kooyman then took off her swimsuit bottoms, restrained her arms and raped her as she yelled "just stop, just stop." Kooyman, she told police, only laughed.
A nurse who later examined the woman described injuries to the woman's genitals as the worst she had ever seen.
The woman told police she thought Kooyman drugged her while the two were drinking at a bar, according to jail documents. Upon searching Kooyman's home, police wrote, they discovered a vials full of clear liquid believed to be the date-rape drug GHB.
But, Kooyman's lawyer said, when the liquid was tested it was revealed to be water.
The victim's toxicology report also did not indicate she had consumed GHB at the time of the alleged assault.
Kooyman was originally charged with rape, object rape and forcible sodomy, first-degree felonies, and two lower felony counts of possession of a firearm by a restricted person.
If convicted of any of the rape charges, he could have faced up to life in prison.
"It turned out, as we did some further investigations, the evidence that we had was compromised," Salt Lake County District Attorney Sim Gill said. "There was not a reasonable likelihood of success at trial."
Kooyman, who has been a registered sex offender since 2002, was convicted by a 3rd District Court jury of second-degree felony forcible sexual abuse for an attack on his housekeeper that year. According to testimony, Kooyman drugged her before assaulting her.
He was sentenced to a year in jail, and his name was added to Utah's sex offender registry.
Kooyman, who has been in custody since the alleged June assault, was ordered to be released Friday.