PHOENIX » A female guard was sexually assaulted by a male inmate at an Arizona prison in the second such attack on an employee in the last 18 months, corrections officials said.

The April 13 attack at the state prison complex in Yuma was only revealed by the state Department of Corrections late Friday after inquiries by The Associated Press.

When asked why the department didn’t acknowledge the attack earlier, corrections spokesman Andrew Wilder said the agency was focused on investigating the assault.

"Our priority is to have a full and thorough investigation of the facts and make sure that is conducted to ensure that justice is served for the victim in the matter," Wilder said.

The assault follows the January 2014 rape of a female teacher at another prison that brought intense criticism of the Arizona prison procedures for safeguarding corrections workers. The bare-bones details of that attack were announced by the department shortly after it occurred, and other inmate assaults on staff often are announced by the department in news releases.

The most recent attack occurred in an office while the corrections officer was meeting with inmate Fernandes Masters, Wilder said.

"The inmate attacked the officer during a scheduled meeting in the housing unit office, and no weapons were involved," Wilder said. "The officer called for assistance, staff in the housing unit immediately responded to the location and pulled the inmate away from the officer and secured him."

The officer, who was not identified, was treated at the prison before being taken to a hospital for evaluation. She was later released, but Wilder did not provide additional details of injuries, the assault or how the officer called for assistance. He also wouldn’t say whether any security procedures were violated or will be changed as a result of the incident.

A criminal investigation by prison officials is ongoing, he said.

"Pending the outcome of this investigation and due to the nature of this violent attack, the department intends to pursue criminal charges against inmate Masters, including sexual assault, kidnapping and attempted murder," Wilder said.

The Yuma County Sheriff’s Office and police in San Luis, where the prison is located, were unaware of the attack when contacted by the AP on Thursday. Roger Nelson, chief criminal deputy at the Yuma County Attorney’s Office, said he also was unaware of the incident. But Wilder said Saturday that prison investigators have consulted with a deputy county prosecutor.

Masters pleaded guilty to first-degree murder in 2007 in a plea agreement that came after prosecutors dropped the death penalty, Maricopa County Superior Court records show. He was sentenced to life without the possibility of parole.

According to the original criminal complaint provided Saturday by the Maricopa County Attorney’s Office and a Glendale police probable cause statement, Masters admitted stabbing his stepfather to death so he could steal his cellphone and vehicle to sell for drug money on Dec. 22, 2004.

Police discovered the crime after a fire was reported in the apartment of the stepfather, Warren Taylor. Masters told detectives he used three different knives to kill him.

Prison records show Masters, 31, has repeatedly been disciplined and has a history of assaults. Online Corrections Department disciplinary records show at least three previous assaults, including one on a prison staff member, and an indecent-exposure incident.

Emilio Ruiz, a board member for the state prison guard’s union who represents staff at the Yuma prison, declined to comment Saturday. Calls to the executive president of the Arizona Correctional Peace Officers Association were not immediately returned.

The teacher who was raped last year is suing corrections officials for failing to protect her. She was alone in a classroom giving assessment tests to inmates in a sex offender unit at the Eyman prison in Florence when she was attacked.

The state workplace safety agency opened an investigation after details of the teacher’s assault were reported by the AP in June. In January, the agency levied fines of $14,000 for two violations of workplace-safety rules, but the Department of Corrections is appealing.

Inmate Jacob Harvey, who was less than a year into a 30-year sentence for a brutal home-invasion and rape, lingered after other inmates left the room on Jan. 30, 2014, then repeatedly stabbed the teacher with a pen before raping her, according to investigative reports and the teacher.

Harvey, now 21, has pleaded not guilty to sexual assault, kidnapping and assault with a deadly weapon charges and awaits trial. His public defender, Paula Cook, has repeatedly declined to comment on the charges.