The agents were stationed at a Border Patrol post named after the last agent killed. (Creative Commons/ Bill85704)

The agents had gone on horseback to investigate a sensor which had been triggered near Naco. It is not clear if the agents returned fire and the incident is under investigation by the U.S. Border Patrol, the Houston Chronicle reported. No suspects have been identified.

SEE ALSO: The Status At The U.S. Mexico Border

The name of the agent who was killed has not been released pending notification of relatives. The injured agent was airlifted to a hospital to be treated for wounds which are not life-threatening. Another agent who was with them was not injured.

The agents were stationed at Brian Terry Station, formerly Naco Station, in Brisbee, 210 miles south of Phoenix. The station was renamed after the last border agent killed, whose death sparked a Congressional investigation which resulted in Attorney General Eric Holder being held in contempt.

SEE ALSO: House Holds Attorney General Eric Holder In Contempt

From USA Today:

In September, Naco Station was renamed the Brian Terry Station in honor of the slain Border Patrol agent. Tuesday's shootings came two weeks after the release of a federal inquiry -- into the botched Operation Fast and Furious gun trafficking investigation -- prompted by Terry's 2010 murder. Two weapons found at the scene of Terry's slaying were linked to an undercover federal investigation that allowed about 2,000 guns to fall into the hands of Mexican drug cartel enforcers and other criminals.

SEE ALSO: Protecting America's Borders

The FBi is also investigating the shooting.

Click here for more coverage of border security.

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[View the story "Tragedy bring border security to the forefront." on Storify]