A group of Asian men set off alarm bells in U.S. counterintelligence circles last week by showing up outside the entrance to a U.S. strategic missile base in Wyoming.

Between eight and 10 people suspected of being Chinese nationals drove up to the entrance outside F.E. Warren Air Force Base, one of three strategic nuclear missile bases in the United States.

According to defense officials and a base spokeswoman, the group asked to use the rest room at the base’s visitor control center. They then began asking questions about photos of Air Force command leaders posted on a "command board" at the entrance facility.

The suspicious visitors then asked to photograph display missiles near the entrance to the base, and were denied.

The base, home to the 90th Missile Wing that includes a group of Minuteman III nuclear missiles, is located about three miles west of Cheyenne, Wyo.

One security official said the suspicious incident on Sept. 3 appeared to be part of a Chinese intelligence collection operation or perhaps a training exercise for intelligence personnel. Another theory is that the group was part of the population of Asian guest workers residing in other parts of Wyoming or the west.

U.S. intelligence officials have said Chinese intelligence agencies conduct aggressive spying activities against U.S. military facilities and have been known to case the Pentagon’s strategic missile defense base at Fort Greeley, Alaska.

Air Force Staff Sgt. Torie Sabarese said that, despite being told that photographs of the base entrance were prohibited, base officials did not regard the incident as a security threat.

"There wasn’t any need for alarm," she said, without elaborating.

Sabarese described the visitors as "Asian" and said local authorities were not called to investigate.

A defense official said the incident might have been "Chinese reconnaissance" operation to spy on a U.S. strategic missile installation.

The incident followed three recent Chinese long-range strategic missile tests.

"I think it is significant that it happened the same time that the PRC is testing missiles," the official said, noting that the inquiries about the command leadership also were suspicious.

The base is headquarters for missile launch facilities in southeast Wyoming, western Nebraska, and northern Colorado.

The base is also the location of the 20th Air Force, which is in charge of all Air Force ICBMs.

The 20th is under the Air Force Strike Command.

A former senior U.S. counterintelligence official said the problem of Chinese intelligence collection has been largely overlooked by the FBI, which is in charge of counterintelligence against foreign states.

During the Cold War, for example, the Soviet military used shipping containers on rail cars to conduct electronic spying operations near strategic missile bases in Wyoming and Nebraska.

"The Bureau is hopelessly outgunned [by Chinese intelligence] in terms of numbers," the former official said. "They just don’t do much to counter them."

As for monitoring U.S. military facilities, during the 2003 Iraq war, Cuban intelligence operatives monitored U.S. military bases in the United States and abroad and provided the data to Iraq, as well as to the governments of China and Russia, the former official said.

The Soviet KGB during the Cold War was also caught setting up electronic eavesdropping posts in the Southwest United States near military bases, including the Army intelligence post at Fort Huachuca, Ariz. after crossing the U.S.-Mexican border.

"There is a long history of communist intelligence service doing wartime intelligence collection that has simply gone unrecognized and, for diplomatic or political reasons, denied by the FBI," the official said.

"I have every reason to believe, based on that history, that the Chinese are doing the same thing and monitoring strategic facilities."

Regarding Chinese signals intelligence collection against the U.S. military, one team of agents from the 3rd Department of the People’s Liberation Army, which conducts electronic spying, was detected spying on U.S. military operations in Iraq during the 1991 Persian Gulf War, the former official said.

China is known to conduct some of most aggressive intelligence collection activities against U.S. military and defense facilities.

For example, on Feb. 26, 2004, two Chinese diplomats were stopped by security guards outside Los Alamos National Laboratory, in New Mexico, as part of what U.S. officials said was an intelligence mission.

The diplomats from China’s Los Angeles consulate were identified in an incident report as Hua Yu and Bo Lai, and were stopped after their rented car sped past a security post. The road they were traveling on was closed to the public and the two men were escorted out of the area. The road led to two sensitive facilities: Los Alamos’ Critical Assembly Facility known as Technical Area-18, and the Plutonium Research Facility, known as Technical Area-55. Both are used for classified nuclear weapons activity.

Chinese military intelligence agents also have been detected conducting surveillance of Fort Greely, Alaska, home to one of two U.S. strategic missile defense interceptor bases that are considered major targets of Chinese intelligence gathering. The second base is located in California.

U.S. officials have said Chinese spying in the United States is considered very active and involves aggressive efforts to recruit Americans with access to secrets.

In recent years, intelligence officials have said China is increasing its electronic intelligence gathering in the United States through a combination of Internet solicitation and electronic eavesdropping, mainly from equipment inside China’s embassy and its diplomatic and commercial outposts.