Four Mexican army soldiers entered southern Arizona and pointed their rifles at a U.S. Border Patrol agent early this week, the Border Patrol said.

The incident Sunday was the Mexican military's 43rd incursion across the U.S. border since October, the agency said. However, it was unusual because firearms were involved. The Border Patrol and the Mexican government are investigating, Border Patrol spokesman Mike Scioli said.

Details remain sketchy, but the incident occurred at 2 a.m. on the Tohono O'odham Indian Reservation about 50 miles southeast of Ajo. The incident took place just north of the border in sight of the new border fence.

The soldiers held their weapons on the agent for several minutes until he identified himself in Spanish, whereupon they lowered their guns and walked back across a gap in the fence, Scioli said.

In Washington, D.C., State Department spokesman Gonzalo Gallegos said the encounter "stemmed from a momentary misunderstanding as to the exact location of the U.S.-Mexican border."

The Mexican army has told the Border Patrol it knew of an incident involving its soldiers, but it did not confirm the details. In years past, the Border Patrol has arrested Mexican soldiers who crossed the border, then typically released them back to Mexico.

Associated Press contributed to this article.