EL PASO, Texas (Border Report) — Guards used a chemical agent to quell a group of unruly inmates at the Biscoe Unit state prison on Nov. 7 in Dilley, Texas, the Texas Department of Criminal Justice confirmed.

A TDCJ spokesman said the guards had ordered inmates to go to their cells, and they refused. The inmates — all U.S. citizens — then broke a window and set fire to a mattress.

The guards deployed a riot control agent to gain control of the situation, and the inmates returned to their cells.

As part of Operation Lone Star, Gov. Greg Abbott directed the TDCJ to move prisoners from the Briscoe Unit to make room for migrants with low-level offenses. However, the Briscoe Unit also houses dozens of U.S. citizens charged with human smuggling.

The small riot took place in a building separate from where migrants stay, the TDCJ spokesman said.

There were no staff injuries.

Operation Lone Star launched on March 6, sending Department of Public Safety troopers and the Texas National Guard to “high-threat” areas along the border. The purpose of the operation was “to deny Mexican Cartels and other smugglers the ability to move drugs and people into Texas,” a March release from the governor’s office said.

Nexstar’s Maggie Glynn contributed to this report.