BOISE, Idaho (AP) — After a three-year investigation into allegations of possible criminal civil rights violations at Idaho's largest private prison, the U.S. Department of Justice is declining to prosecute any current or former guards with Corrections Corporation of America.

U.S. Attorney Wendy Olson made the announcement Monday, saying the FBI's investigation into inmate-on-inmate assaults at the CCA-run Idaho Correctional Center south of Boise was detailed and covered multiple incidences. Olson says that while the assaults at the prison have been problematic, prosecutors didn't believe they would be able to prove elements of a federal crime beyond a reasonable doubt.

Under federal criminal civil rights law, prison guards commit a crime if they willfully fail to stop an assault or are deliberately indifferent to an inmate who is in need of medical care. In a prepared statement, Olson said the investigation focused on whether correctional staffers had "actual knowledge of a substantial risk of serious harm to the inmate."

That's a tough standard, Olson said.

"We determined that under the circumstances, none of these assaults were incidents where we could prove the elements of a federal offense beyond a reasonable doubt. In such situations, we are obligated to decline prosecution. We do so here," Olson said.

CCA spokesman Steve Owen said the company cooperated fully with investigators and that the safety of inmates, staffers and communities is a top priority.

"We appreciate the Department's thorough review of this matter as well as the outcome," Owen said.

Olson, the U.S. Attorney in Idaho, confirmed in November 2010 that the Department of Justice investigation was underway after The Associated Press published a story and surveillance video of an attack on former ICC inmate Hanni Elabed. Elabed was attacked in January 2010 moments after prison staff reassigned him to a cellblock with inmates he previously identified as being involved in peddling contraband with a staffer.

The harrowing security video showed that a group of guards watched for several minutes as Elabed was attacked by another inmate. At one point, the footage shows Elabed banging on the window to a guard station and pleading for help to no avail. The guards continued to watch as Elabed was knocked unconscious, stomped and kicked; they didn't intervene even when Elabed's attacker sat down for a minute to catch his breath before continuing to beat the unconscious victim.

Elabed was left with permanent brain damage from the attack. He sued CCA in federal court and reached a settlement with the company; he was also granted a medical release from prison.

At the time, Olson said FBI agents were focusing on whether ICC staff had violated the civil rights of Elabed and other inmates at the prison, which is operated by the Nashville, Tenn.-based CCA under a $29 million contract with the state of Idaho.

The investigation was focused solely on the Idaho prison and not any of the other prisons operated across the country by CCA.

Several inmates at the Idaho Correctional Center have sued CCA in federal court alleging that the company's management contributes to high rates of violence at the prison. Those civil rights lawsuits are a more appropriate vehicle for addressing the assaults that the investigation examined, Olson said.

She also noted that the criminal investigation did not cover recent admissions by CCA that staffers at the Idaho Correctional Center did not work all of the hours billed to the state of Idaho.