By Lizbeth Diaz and David Schwartz
MEXICO CITY/PHOENIX (Reuters) - Mexican troops arrested two men on Wednesday suspected of involvement in the killing of a U.S. Border Patrol agent shot dead in Arizona while responding to a tripped ground sensor, Mexican security officials said.
The agent who died was among three who were patrolling on foot about 5 miles north of the international border when gunfire erupted well before daybreak on Tuesday. A second agent was also wounded while the third, a woman, was unharmed.
The agents involved in the incident had been patrolling in an area near the border town of Naco, well-known as a corridor for smuggling, and the Cochise County Sheriff's department has said that tracks were found heading south after the shooting.
The two suspects detained in Mexico were arrested in a Mexican military operation in the city of Agua Prieta, in Mexico's northern Sonora state, a few miles (km) from the spot where Nicholas Ivie, 30, was shot dead, a Mexican Army officer, who declined to be named, told Reuters.
A Mexican police official in Naco, across the border from the Arizona town of the same name, confirmed the arrests, which occurred in the early hours of Wednesday.
The killing marked the fourth death of a Border Patrol agent in a violent confrontation 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.
The violence drew sharp words from Republican Governor Jan Brewer, a vocal foe of President Barack Obama's administration on immigration. She said it should lead to anger over "the federal failure and political stalemate that has left our border unsecured and our Border Patrol in harm's way."
Authorities on the U.S. side of the border combed rugged terrain looking for clues into the shooting near Naco, which remains a smuggling corridor despite the construction of a tall, steel fence along the border.
"We're still out there collecting evidence," said Brenda Nath, a Federal Bureau of Investigation spokeswoman in Phoenix, declining to say what had been found so far.
Cochise County Sheriff's spokeswoman Carol Capas could not immediately comment on the arrests in Mexico, saying that she had not received any information about them. Nath also declined to comment on word of the arrests.
Ivie, a border agent since 2008, was found dead at the scene. The wounded agent, who has not been publicly identified, sustained non-life-threatening injuries and has been released from the hospital, the border patrol said on Wednesday.
The agents had been responding to a sensor, which picks up 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.
The agents 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 U.S. operation to track guns smuggled to Mexico.
Two Border Patrol agents were killed last year in an accident during a car chase with smugglers near Phoenix.
(Editing by Cynthia Johnston and Sandra Maler)