Cassell was the co-author of motion to dismiss the case based on alleged "improper use of compelled testimony."
The motion, filed Monday, claims that prosecutors relied on numerous statements obtained by the West Valley City police department that Cowley was "ordered to provide under a direct threat of job loss."
The motion says prosecutors are prohibited from using such compelled statements, yet the "state investigation and prosecution team were deeply involved in reviewing those statements."
The motion asks the judge to hold a hearing at which it would be the prosecution's burden to prove "they made no use whatsoever" of Cowleys' compelled statements.
The defense motion says they anticipate that at the end of such a hearing, the state will be unable to satisfy its burden.
"Accordingly, at that time Detective Cowley will ask for and be entitled to the dismissal of the charge against him," the motion states.
Cowley, 33, will be back in court Aug. 5 for another scheduling conference.
He could face up to 15 years in prison, if convicted.
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.
Danielle Willard proceeded to back out of a parking spot as the two detectives approached her car.
Cowley has said he was in the car's path and believed his life was in danger, that the car's rear bumper struck his leg . He fired one shot, which hit Willard, 21, in the head.
Salmon fired after Cowley, but missed Willard.
Salt Lake County District Attorney Sim Gill has said the evidence shows Cowley was to the side of the car, and his life was not in jeopardy.
Monday's hearing will likely be a brief one for Cowley, who was charged with last month in the fatal shooting.
Cowley was fired by West Valley City but for allegedly mishandling evidence, not for shooting Willard.
Earlier this month, his case was assigned to 3rd District Judge Robin Reese.