(CNN) -- A Border Patrol agent who was shot and killed Tuesday near Naco, Arizona, was responding to a sensor that had gone off at the border, a law enforcement official said.
Another agent sustained injuries that are not considered life-threatening, said Brent Cagen, a Border Patrol spokesman.
The shootings happened in the southeastern part of the state, near the Mexican border.
The FBI is conducting a joint investigation with the Cochise County Sheriff's Office, said Jennifer Giannola, public affairs specialist at the bureau's Phoenix division.
Lanny Breuer, assistant attorney general for the Criminal Division of the Justice Department, said investigators are at the scene.
"Every time that a law enforcement person is either killed or shot or injured in the line of duty, we have to take a moment and think of our families and think of the heroes involved," Breuer said.
"Our hearts and prayers go out to the families," said Breuer.
The Border Patrol agent killed Tuesday is the 14th killed in the line of duty since 2008, including three this year, the agency said.