PHOENIX — A female guard was sexually assaulted by a male inmate at an Arizona prison last month 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.

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.”

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 scant 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, and Wilder said a criminal investigation by prison officials is ongoing.