UPDATE 1-Mexican troops arrest two in killing of U.S. border agent - officials

Lizbeth Diaz and David Schwartz

* Shooting killed agent Nicholas Ivie, wounded another

* Agents had been responding to tripped ground sensor

* Tracks found heading south after the shooting

(Recasts with Mexican authorities arresting suspects)

MEXICO CITY/PHOENIX, Oct 3 (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 (8 km) 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)