A US border patrol agent has been shot dead near Arizona’s US-Mexico border in an area known for drug-smuggling, authorities say.
Another was wounded when the two responded to a tripped ground sensor in early on Tuesday.
Nicholas Ivie, 30, was the first agent killed since 2010, when a death led to a review of a botched US-run operation.
In Operation Fast and Furious, agents lost track of illegal guns from Arizona allowed into Mexico to target dealers.
Authorities said Mr Ivie and a fellow agent had been on foot near Naco, Arizona, about five miles (8km) north of the border, when gunshots rang out around 02:00 local time (09:00 GMT).
The second agent, who has not been identified, was airlifted to hospital and is being treated for non-life threatening injuries.
Speaking at a news briefing, authorities declined to say whether any weapons had been seized at the scene.
“As they were walking up the trail, they reported taking gunfire,” Cochise County Sheriff’s spokeswoman Carol Capas told Reuters.
She said it was not known who shot the agents.
Arizona’s Republican Governor Jan Brewer, who has previously criticised the Obama administration’s immigration policy, said in a statement: “Arizonans and Americans will grieve, and they should.
“There should be anger, too. Righteous anger – at the kind of evil that causes sorrow this deep, and at the federal failure and political stalemate that has left our border unsecured and our border patrol in harm’s way.”
In December 2010 Agent Brian Terry was killed in a shootout with Mexican bandits. Later, his death was linked with Operation Fast and Furious, which was tracking illegal guns.
Two of the illegal weapons were found at the scene of the border agent’s murder. Fast and Furious ended in early 2011.
No arrests were made in connection with Terry’s death, but 14 officials were recently cited for possible disciplinary action in a US justice department investigation into the handling of the operation.
In a statement, Republican Senator Chuck Grassley said: “Because of Operation Fast and Furious, we’ll wonder for years if the guns used in any killing along the border were part of an ill-advised gun-walking strategy.”