By David Schwartz

PHOENIX (Reuters) - Friendly fire was the likely cause of the death of a Border Patrol agent shot in Arizona near the Mexican border this week, according to strong preliminary indications, the FBI said on Friday.

Nicholas Ivie was one of three agents patrolling on foot about five miles (eight kilometers) north of the international border before daybreak on Tuesday when gunfire erupted as the agents responded to a tripped ground sensor, authorities have said.

A second agent was wounded in the incident near the border town of Naco, an area well known as a smuggling corridor. The third agent was unharmed.

"While it is important to emphasize that the FBI's investigation is actively continuing, there are strong preliminary indications that the death of United States Border Patrol Agent Nicholas J. Ivie and the injury to a second agent was the result of an accidental shooting incident involving only the agents," the FBI said in a statement.

Ivie was the fourth Border Patrol agent to die in violent circumstances in less than two years in Arizona. His death heightened concern about border security in a state at the forefront of the national immigration debate.

U.S. authorities had previously released scant details about the circumstances of the shootings. A local sheriff's official had said that the Border Patrol tried to arrest two suspects but they fled to Mexico.

Mexican officials said two men were later arrested in a military operation near the city of Agua Prieta, a few miles across the border from the shooting scene. U.S. authorities have declined comment on those arrests.

U.S. Homeland Security Secretary Janet Napolitano, a former Arizona governor, held talks with law enforcement officials on Friday at the Border Patrol station where Ivie worked.

Kevin Goates, an Ivie family spokesman, declined to comment on the FBI announcement of the preliminary findings in the case.

He said Napolitano had met with Ivie's wife, Christy, and other family members for about 40 minutes at her home in southern Arizona.

(Additional reporting by Mark Hosenball, Writing by Alex Dobuzinskis; Editing by Cynthia Johnston and Christopher Wilson)