A US Border Patrol agent was shot dead and another wounded when they came under fire in a drug smuggling corridor in Arizona near the border with Mexico..
Authorities said three agents were on foot about 8km north of the border in rocky terrain when gunfire erupted well before daybreak.
"As they were walking up the trail, they reported taking gunfire," Cochise County Sheriff's spokeswoman Carol Capas said. "We have unknown suspect or suspects at this point."
The shooting marked the fourth conflict-related death of a Border Patrol agent in Arizona in less than two years and reignited concerns about border security in a state that is already at the forefront of the national immigration debate.
"Flags will be lowered in honour of the slain agent. Elected officials will vow to find those responsible. Arizonans and Americans will grieve, and they should. But this ought not only be a day of tears," Republican Governor Jan Brewer said.
"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," added Brewer, a vocal foe of President Barack Obama's administration on immigration.
Brewer, citing what she described as a federal failure to secure Arizona's southern border, signed a broad immigration crackdown into law in 2010 to try to crack down on the flow of illegal immigrants into the state where an estimated 360,000 undocumented people live.
Critics of the law, which includes a requirement that police check the immigration status of anyone they stop and suspect of being in the country illegally, have said it could lead to racial profiling.
The shooting took place near the border town of Naco, southeast of Tucson, which remains a corridor for smuggling marijuana and people, despite the construction of a tall, steel fence along the border.
"We need to redouble our efforts to secure the border and ensure the safety of Border Patrol agents," US Democratic Representative Ron Barber, who represents the southern Arizona district where the shooting occurred, said in a statement.
Sheriff's deputies were called to the scene at 1:33am local time and found one agent dead and another with non-life-threatening injuries, Capas said. A third was unharmed. FBI agents were also investigating.
Across the border from Naco in a Mexican town of the same name, Mexican police said a team of soldiers and federal and local police was searching for a suspect or suspects in the case. "We have no information about anybody being detained," said a Naco police officer who declined to be named.
The Border Patrol identified the slain agent as Nicholas Ivie, 30, who was originally from Utah and had worked for the agency since 2008.
President Obama called Ivie's family and expressed his sadness for the family's loss and his gratitude for Ivie's service to his nation, according to a White House statement. Obama said his administration was doing everything it could to locate those responsible.
The agents had been responding to a sensor, which picks up on movement or vibrations in areas authorities suspect are used by drug traffickers and illegal immigrants. When an alert is triggered, agents have the option to respond.
Capas said the agents who were shot were assigned to the Brian A. Terry Border Patrol Station, named after an agent whose 2010 death in the line of duty in Arizona borderlands was linked to a botched US operation to track guns smuggled to Mexico.
In that case, two guns tracked by the Bureau of Alcohol, Tobacco, Firearms and Explosives in the "Fast and Furious" sting operation, which let weapons slip into Mexico, were retrieved from the spot where Terry died in a shoot-out with bandits. It was unclear if the weapons were used in his murder.
Two Border Patrol agents were killed last year in an accident during a car chase with smugglers near Phoenix.