TUCSON, Ariz. (AP) - Unknown assailants threw rocks at a National Guard border observation post in southern Arizona, breaking two windows on a Guard vehicle, but no one was injured, a Border Patrol spokesman said Tuesday.

Border Patrol agents searched for the rock-throwers following Sunday night's incident but only found their tracks, which led into Mexico, said Sean King, a spokesman for the Border Patrol's Tucson sector, which covers most of the Arizona-Mexico border.

The Arizona National Guard will assign an officer to investigate what happened as it does with any incident in which its soldiers faced potential danger, said Maj. Paul Aguirre, a spokesman for the Arizona National Guard.

"I think the longer we are involved in the mission there is a greater possibility that incidents like this will take place," Aguirre added.

The soldiers involved in the incident were among the 6,400 National Guard members sent to the four southern border states to perform support duties that tie up immigration agents, who then have more time to catch illegal immigrants.

They were part of an entry identification team, whose job it is to watch the border and report illegal crossings to Border Patrol agents.

Assaults by rock-throwers targeting border agents are common on the Arizona-Mexico border. King said it's a daily occurrence in some areas, though not on the Indian reservation where Sunday night's attack occurred.

King said some Arizona agents have been seriously injured, including one who lost use of an eye. "It's life-threatening," King said.

Sunday's incident was the second this month involving a National Guard border observation team.

In an incident that has drawn national attention, four National Guard members pulled back from an observation post on Jan. 3 after they were approached by several armed men.

The Guardsmen called Border Patrol agents, who tracked the armed men back to the border, but weren't able to find them. No shots were fired.