| SAN LUIS POTOSI, Mexico
SAN LUIS POTOSI, Mexico Mexican drug gang hitmen were behind the shooting of two U.S. immigration and customs agents on a major highway, the governor of the state where the men were attacked said on Wednesday.
One Immigration and Customs Enforcement (ICE) agent was killed and a second was wounded in the attack on Tuesday, which came against a backdrop of spiraling violence between Mexican security forces and drug cartels.
"There was an attack where drug gang members ... tried to kill two U.S. officials on a federal highway," San Luis Potosi state governor Fernando Toranzo said in a radio interview.
United States officials have warned the cartels in recent weeks against any attempt to bring their brutal tactics across the border. If a gang responsible for the attack on the agents is identified, U.S. intelligence work is likely to focus on trying to catch its leaders and cripple its operations.
Washington has provided funds, training and political support for Mexican President Felipe Calderon's army-led strategy to crush the cartels and U.S. intelligence is believed to have played a major role in the killing or capture of several top leaders in recent years.
U.S. law enforcement agencies said they were working with Mexican authorities to investigate the attack. A State Department spokeswoman said she could not confirm if the agents were specifically targeted.
The agent who survived the shooting told a doctor at a local hospital that two vehicles overtook their armored SUV on the highway north of Mexico City and forced them off the road.
After one of the agents opened his door, someone forced a machine gun into the vehicle and sprayed them with bullets, said Felix Hernandez, medical director at the hospital, who interviewed the injured agent.
The other agent, Jaime Zapata, was shot five times in the stomach and lower body and went into cardiac arrest on the way to hospital, where he was pronounced dead, Hernandez said.
The survivor was shot in the leg and was transferred to the United States once he was stabilized.
Mexican officials had said the agents may have been ambushed at a fake military checkpoint, which are sometimes set up by cartels to trap rivals.
The flag at the U.S. embassy in Mexico City, where both agents were assigned, was lowered to half mast on Wednesday.
More than 15,000 people were killed in drug violence in Mexico last year as fighting between the cartels and security forces spread to cities once considered distant from the front lines of the drug war, including Monterrey.
But attacks on U.S. officials are rare and the targeting of U.S. agents would be a major escalation of the war.
Officials said the agents were returning to Mexico City after meeting other U.S. personnel in San Luis Potosi, not en route between Mexico City and Monterrey as earlier reported.
They were unarmed because U.S. law enforcement officers are not authorized to carry guns in Mexico.
Two U.S. citizens and a Mexican linked to staff at the U.S. consulate in Ciudad Juarez were killed in March last year, prompting the State Department to tighten security at its diplomatic missions in northern Mexico.
Officials have not said which of Mexico's drug cartels may have been behind the latest attack, although many suspect the Zetas, a group led by renegade Mexican soldiers and widely seen as the most brutal of the gangs.
"The United States is going to have to make a quick evaluation of the security of its officials in Mexico. Was it wise for these agents to be traveling in a shiny black vehicle along a highway controlled by the Zetas?," said Fred Burton of intelligence consultancy Stratfor.
San Luis Potosi governor Toranzo said authorities suspected the Gulf cartel and Zetas were active in the region, which is arid and dotted with cactii.
"We know that organized crime is active in our state and there is a territorial dispute going on," he told reporters, adding that it was too early to say who was responsible.
(Additional reporting by Miguel Angel Gutierez, Anahi Rama, Jason Lange, Dave Graham and Mica Rosenberg in Mexico City, Robin Emmott in Monterrey, Jeremy Pelofsky in Washington and Tim Gaynor in Phoenix; Writing by Robert Campbell; Editing by Kieran Murray)