One U.S. Border Patrol agent was killed and another wounded Tuesday in a shootout with unknown gunmen near the U.S.-Mexican border in Arizona’s Cochise County.

The gunfight took place around 2 a.m. in the town of Naco, the National Border Patrol Council said.

Three agents arrived at the site on horseback after a ground sensor in the border zone was activated.

The Border Patrol, which did not reveal the name of the dead agent, said that it is still investigating who and how many people fired at the federal agents.

The agent who was wounded was transported to a hospital in stable condition, while the third agent was unhurt in the shootout.

The incident in Naco is being investigated by the FBI and the Cochise County Sheriff’s Office.

The agents were assigned to the Border Patrol station in Naco, a post named for Brian Terry, an agent who was killed during a fight with smugglers in December 2010 near Nogales, Arizona.

The Arizona border is considered one of the main crossing corridors for undocumented immigrants and illegal drugs along the border with Mexico.