Federal prosecutors have decided to not pursue criminal charges against three Fullerton police officers whose violent encounter with Kelly Thomas five years ago resulted in the homeless, mentally ill man’s death.

Department of Justice officials said Monday that prosecutors have determined that they are unable to prove that former officers Manuel Ramos, Jay Cicinelli and Joseph Wolfe violated Thomas’ civil rights.

The now-closed federal civil rights case was the last remaining investigation into 37-year-old Thomas’ death, which drew national attention after security video of his deadly encounter with police at the Fullerton Transportation Center went viral.

Thomas’ father, Ron Thomas, said he wasn’t surprised by the Department of Justice’s decision, given how long the investigation has lasted.

“I didn’t have much faith in it at all,” Ron Thomas said Monday of the federal investigation. “But that is over with now, and I’m moving forward and will continue with my advocacy work.”

Thomas provided a letter he received from the U.S. Department of Justice late last week, in which U.S. Attorney Eileen Decker indicated that the decision had been made to close the federal investigation.

“We understand that you may be disappointed that we cannot prosecute this matter as a federal civil rights crime, but we assure you that the FBI, the United State’s Attorney’s Office and the Civil Rights Division devoted many hours and significant resources to conducting a complete and thorough investigation and prosecutorial review of this incident,” Decker wrote. “Again, please accept our condolences on the loss of your son.”

In the letter, Decker noted that in order to convict the officers involved in Thomas’ death on federal charges, prosecutors would have to prove that they willfully disregarded his civil rights.

“Neither accident, mistake, fear, negligence nor bad judgment is sufficient to establish a willful federal criminal civil rights violation,” Decker wrote.

Interim Fullerton Police Chief David Hinig said that his department was notified of the DOJ’s decision on Monday morning. Hinig said his department appreciated the deliberation that went into the DOJ’s decision, but declined to comment further.

Thomas, a transient know to frequent Fullerton and other surrounding cities, died five days after police took him into custody while investigating reports that he was stealing small personal items.

A city security video shows several officers threatening Thomas, who screamed for his father to help him as he was struck by the officers. The violent encounter left Thomas unconscious, having suffered a compressed trachea and broken bones in his face.

A 2013-14 criminal trial in state court, prosecuted by Orange County District Attorney Tony Rackauckas, ended with Ramos and Cicanelli found not guilty and charges against Wolfe dropped. All three men were fired from the department after Thomas’ death.

A year later, a civil trial brought by Ron Thomas was called off at the eleventh hour when the father accepted a $4.9 million settlement from the city of Fullerton. Kelly Thomas’ mother, Cathy Thomas, had already accepted a $1 million settlement.

Contact the writer: semery@scng.com