An alert plainclothes U.S. Border Patrol agent who previously served in the Marine Corps foiled a brazen attempt by 13 illegal immigrants and two suspected U.S. smugglers to enter the United States by asking the driver a simple question that every Leatherneck knows: When is the Marine Corps' birthday?

On Nov. 10, 1775, a Corps of Marines was created by a resolution of the Continental Congress, according to the U.S. Marines' website. Formal recognition of the event, however, did not occur until 1921. Prior to that year, it's unclear whether any celebration was held.

But every Marine is taught to remember that historic date -- and the incorrect answer tipped off Border Patrol agents at the Campo Border Patrol Westbound I-8 checkpoint on March 14 near Pine Valley, Calif.

Another agent later noticed that passenger Jose Guadalupe Ceja Jr., a suspected smuggler, seemingly did not understand English. Ceja and the driver of the white van both wore Marine uniforms bearing nametags reading "Lopez."

The use of Marine disguises appears to be one of the first cases of smugglers and immigrants posing as U.S. military. Mexican smugglers often don that country's military uniforms during attempts to get their illegal loads past authorities. In a 2006 incident that strained U.S.-Mexico relations, traffickers dressed as Mexican soldiers crossed the Rio Grande and were seen helping suspected drug smugglers elude U.S. law enforcement during a chase.

"This effort is an example of the lengths smugglers will go to avoid detection, and the skilled and effective police work and vigilance displayed every day by Customs and Border Protection personnel," U.S. Customs and Border Protection officials said in a statement to FoxNews.com on Tuesday.

Former Marine Capt. David Danelo, a senior fellow at Foreign Policy Research Institute in Philadelphia who has authored a book about the U.S.-Mexico border, says smugglers had the unfortunate luck of running into well-trained Border Patrol agents with military experience.

"Should we punish these guys by sending them through four years of basic training?" he told the Associated Press. "The troubling reality and the real question here is, has this ever succeeded before? That's an answer we just don't know."

Several unanswered questions do remain, including how the suspects acquired the uniforms and their intentions once in the United States.

The Naval Criminal Investigative Service -- the investigative arm of the Navy, which includes the Marine Corps -- has teamed up with the Border Patrol to find out.

"If people are pretending to be Marines for criminal reasons, we'll want to know why," said Ed Buice, spokesman for the Navy's investigative arm, known as NCIS.

Buice declined to discuss details of the investigation.

"I'm sure they were hoping agents would just see military people in a white van with government plates and just wave them through," Border Patrol spokesman Michael Jimenez told the Associated Press.

According to a criminal complaint filed in U.S. District Court in San Diego, the van with a government plate caught the eye of a Border Patrol agent identified only as S. Smith who was driving an unmarked vehicle down the interstate last week.

The van with the words "U.S. Government, For official use only" on the license plate seemed suspicious. One of the numbers also reflected light differently when Smith's headlights shined on it.

Smith then sped up and passed the van to get a better look: The driver was wearing a military uniform and he could see others in the back wearing Marine Corps caps.

Smith called his colleagues at the nearest checkpoint and told them to do a close inspection of the van when it arrived. He followed and asked the driver during the inspection where they were headed.

"Joint Service Base," he replied.

Smith was unmoved, especially after noticing that one of the numbers on the license plate of the van had been altered from a 0 to an 8.

Another agent, identified only by his last name, Robinson, also a former Marine, noticed other anomalies: Some of the group was wearing desert camouflage uniforms and others were wearing urban camouflage uniforms.

Robinson then asked Ceja directly if he was a Marine, and he admitted he was not, according to the complaint.

Agents later tracked down Guadalupe Garcia, another smuggling suspect, who was apparently scouting out agents, at a checkpoint outside Jacumba, Calif., according to the complaint. Marine Corps insignias were found under a passenger seat in his car, authorities said.

Arturo Leyva, another suspect, told authorities he had been asked to smuggle drugs by a man he met at a bar in the border town of Calexico but had backed out, according to the complaint. Authorities say he later ran into the man at a bar across the border in Mexicali and agreed to smuggle immigrants.

He was given a cell phone and called on March 14. He was told a taxi would be taking him from his home in El Centro to Calexico.

The alleged smugglers and immigrants then went to a trailer park, where a man arrived with a military-style duffel bag full of uniforms, the complaint states. The man advised Ceja on how to talk to the Border Patrol, instructing him to say they were coming from Yuma Air Force base.

Marine Capt. Brian Block at the Pentagon said the official attire is the property of service members who buy it when they enter the military and it's up to the individual to keep track of it after they are discharged.

Block said the services strongly encourage military members to maintain control of their uniforms for security reasons, but he acknowledges not everyone heeds the advice.

"You can go into just about any Army-Navy store and pick up old camies if you want to, especially in the San Diego area, where there is a lot of military," he said. "But if you don't have a military ID card, you can't walk onto a base."

Leyva's attorney, Douglas Brown, said his client and the two other U.S. citizens arrested have entered a preliminary plea of not guilty. Ceja's attorney, Martin Molina, declined to comment. Garcia's attorney, Brandon Leblanc, said he could not comment on the case.

The Associated Press contributed to this report.