LUKEVILLE, Ariz. -- U.S. Border Patrol officials still don't know why National Guard troops along the Arizona-Mexico border had to flee a group of armed people. The gunmen fled into Mexico.

The gunmen attacked the U.S. Border Patrol entry site along Arizona's border before retreating back to Mexico on Wednesday night. The troops withdrew safely Wednesday night, and no was injured.

According to the Border Patrol, an unknown number of gunmen overran the Identification Team site, which is manned by National Guardsmen.

The Border Patrol wouldn't say whether any shots were fired.

Officials don't know who the gunmen were or why they approached the border post.

The situation occurred on the 120-mile section of the border between Nogales and Lukeville. The area is known as a major drug corridor and has been a busy one for marijuana seizures.

There are dozens of National Guard entrance identification teams along the Mexican border, including east and west of both Nogales and Sasabe and on the Tohono O'odham Nation.

The troops stand post on hilltops next to army-green tents and serve as extra eyes and ears for the Border Patrol.

With more Border Patrol agents and National Guard troops patrolling the Arizona section of the U.S.-Mexico border, it has become more difficult to smuggle drugs and people across, and "that heightened frustration may have been connected to what took place last night," said Rob Daniels, the spokesman for the Border Patrol's Tucson Sector.

The Border Patrol said its agents have seized a record amount of marijuana from smugglers along the Arizona-Mexico border for the fifth year in a row, confiscating 331 tons of marijuana worth $530 million. The seizure almost equaled the 350 tons seized in California, Texas and New Mexico combined, and the agency praised the Guard for its contribution to making the seizures.

California National Guard helicopters deployed to help monitor the U.S.-Mexico border were grounded Jan. 3 following a crash the day before that injured nine people in the mountains east of San Diego.

The helicopter was bringing a Border Patrol team to remote areas for operations against human smugglers.

Two National Guard soldiers and three Border Patrol agents remain hospitalized. There is no word yet on the cause.

Guard officials said the crash will be investigated by the California National Guard and by a team from the U.S. Army Combat Readiness Center at Fort Rucker, Ala.