Arizona corrections officials say a female guard was sexually assaulted by a male inmate last month in the second such attack on a state prison employee in the last 18 months.

The April 13 attack at the state prison complex in Yuma was only revealed by the Department of Corrections late Friday after inquiries by The Associated Press. Corrections spokesman Andrew Wilder said the agency was focused on investigating the assault.

The assault follows the January 2014 rape of a female teacher at another prison that brought scrutiny of Arizona's procedures for safeguarding prison workers.

The department says the latest attack occurred in a housing unit office while the corrections officer was meeting with inmate Fernandes Masters. The 31-year-old inmate is serving a life sentence for first-degree murder.