WASHINGTON (AP) — The federal prison system has been placed on a nationwide lockdown after two inmates were killed and two others were injured Monday during a gang altercation at a federal penitentiary in Texas.

The incident happened around 11:30 a.m. Monday at USP Beaumont, a federal prison in Beaumont, Texas. The altercation involved members of the violent MS-13 street gang, two people familiar with the matter told The Associated Press. The people could not discuss an ongoing investigation and spoke to the AP on condition of anonymity.

The attack is just the latest example of serious violence within the beleaguered federal Bureau of Prisons. The agency has struggled through a multitude of crises in recent years, including widespread staffing shortages, serious employee misconduct, a series of escapes and deaths.

The lockdown, being instituted at the agency’s more than 120 federal prisons across the U.S., was prompted by fears of potential retaliation and concern violence could spread to other facilities.

During a nationwide lockdown, inmates are kept in their cells most of the day and visiting is canceled.

Because of a spike in coronavirus cases in federal prisons, social visits at nearly every facility have been canceled already.