"I look forward to working with [Lindsay Jarvis, Cowley's attorney] to defend Officer Cowley against what are, in my view, inappropriate charges," Cassell's letter adds.
Cowley was charged with manslaughter last month, a felony that carries up to 15 years in prison.
On Nov. 2, 2012, Cowley and Kevin Salmon, both detectives in West Valley City's neighborhood narcotics unit, observed what they thought was a drug deal. Willard entered a Subaru and backed out of a parking spot as the detectives approached.
Cowley claimed he was in the car's path and believed his life was in danger when he fired one shot striking Willard, 21, in the head. But Salt Lake County District Attorney Sim Gill has said the evidence showed Cowley was to the side of the car. Gill said Sunday that he has immense respect for Cassell, who mentioned to Gill last week that he would be joining Cowley's team.
"Paul and I worked on things together… we continue to work on projects together right now, and we happen to be on different sides [this time]," Gill said.
Cowley has said the car's rear bumper struck his leg. The Subaru brushed Salmon's leg and he fired after Cowley, missing Willard.
Cassell was a federal judge in Utah from 2002 to 2007. He teaches criminal law and is both an attorney and advocate for victims.
In January, he argued a case at the U.S. Supreme Court, asking the justices to uphold a $3.4 million award to a woman who was abused as a girl and whose photos were disseminated as child pornography. The verdict was against a Texas man who viewed the images. But the Supreme Court threw out the award and said federal law limits how much money victims of child pornography can recover.
West Valley City fired Cowley, but not for the shooting. The city said it found he mishandled evidence. Cowley is appealing the termination.
Salmon remains on administrative leave while West Valley City finishes an investigation into his role in the shooting and into problems in the narcotics unit.