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A prominent University of Utah law professor — now representing an immigration agent accused of slamming an inmate's face into a concrete floor — has asked for the criminal case against the agent to be dismissed, arguing in a motion filed Monday that he was improperly charged.

Jon Martinson Jr. was charged last June with deprivation of rights under color of law, accused in court papers of violating the alleged victim's right to "be free from the use of unreasonable force by a law enforcement officer."

But former federal judge and U of U law professor Paul Cassell argues in a motion to dismiss the case that prosecutors have mounted their case under an alleged violation of the Fourth Amendment right against unreasonable force during search and seizure. Cassell claims the facts of the case don't match the charge: The alleged victim, Fabian Maldonado-Pineda, was neither being searched by the U.S. Immigration and Customs Enforcement agent nor being arrested during the incident in question.

Cassell filed the motion on Monday — the same day he announced that he was joining Martinson's defense team.

Cassell told The Tribune that he usually doesn't do criminal defense work, but said he gets involved in cases where people — usually law enforcement officers — are charged and he believes they are innocent.

"Just a couple of days ago, the defense team showed me the evidence in this case," Cassell said. "It looked, in my mind, that the officer was wrongfully charged ... I think this is a case that should not be filed, and I'm hopeful that the government will dismiss it soon."

The former judge argued in the motion that if his client were to be charged with violating Maldonado-Pineda's rights, it should be charged as a violation of the alleged victim's Eighth Amendment right against cruel and unusual punishment. However, federal prosecutors would then have to prove that Martinson acted "maliciously and sadistically for the very purpose of causing harm."

Cassell asked that prosecutors dismiss the case, and if they refuse, he asked the judge to do so.

"The government could attempt to pursue reindictment with a properly instructed grand jury — if it truly thought it could prove that Agent Martinson's split-second decision was made maliciously and sadistically," Cassell wrote. "That he acted with such intent seems highly doubtful, as a properly instructed grand jury would likely conclude... If this case is allowed to move forward, the indictment and the facts of the case will be like two ships passing in the night — having no connection whatsoever."

Cassell would not speculate whether the filed charge was simply a legal error by prosecutors, or if it was done purposefully to avoid having to prove malicious intent at an upcoming trial. The U.S. Attorney's Office declined to comment Monday, saying it will file its response in court.

A federal indictment alleges that in July 2013, Martinson slammed Maldonado-Pineda "onto the concrete floor face first." But the ICE agent's defense attorneys contend that the incident was much more minor. They claim that as Martinson was trying to move Maldonado-Pineda to a different cell, the inmate pulled away repeatedly, and the agent responded with a "hip-toss."

The defense claims Maldonado-Pineda landed on his chest, not his face.

Maldonado-Pineda was in ICE custody, having been convicted of illegal re-entry to the U.S., and was being moved to a different cell because he had been disruptive to other inmates that day, according to Martinson's attorneys.

A five-day jury trial is scheduled to begin on July 13. If Martinson is convicted, he could be imprisoned for up to 10 years and fined $250,000.

Martinson has pleaded not guilty to the charge. He has been suspended from his job indefinitely pending the outcome of the criminal case, according to ICE officials.

This is the second law enforcement officer Cassell has represented in a criminal case in the last year. He also joined the defense team for Shaun Cowley, a former West Valley City police detective charged in state court with manslaughter after the 2012 shooting of 21-year-old Danielle Willard. A 3rd District Court judge dismissed the charge after a preliminary hearing last year, ruling that there was not enough evidence to move the case forward.

Twitter: @jm_miller