Two heavily armed, camouflaged Mexican soldiers crossed 50 yards inside Arizona in January and drew their guns against U.S. Border Patrol agents who confronted them in a tense standoff, according to documents obtained by The Times/Tribune Washington Bureau.

U.S. officials said it was one of nearly two dozen border incursions by Mexican soldiers into southern Arizona in the last four years.

The Jan. 26 confrontation, described in a Border Patrol foreign military incursion report and confirmed in a separate letter from U.S. Customs and Border Protection Commissioner R. Gil Kerlikowske, ended when the Mexican soldiers retreated back over the border after U.S. agents — who also drew their weapons — summoned assistance. The soldiers, who misidentified themselves to border agents, claimed to be pursuing drug smugglers, documents show.

U.S. officials characterized the incident as one of the most serious incursions in recent years. Though gunfire was averted, the 35-minute confrontation underscored the continuing friction between the U.S. and Mexico when it comes to policing the often chaotic and violent Southwest border. It also raised questions among some U.S. officials about whether the Mexican soldiers were chasing drug smugglers, as they claimed, or protecting cartels as they used drug routes to Arizona.