The Mexican navy did not immediately respond to requests for information on the purpose of the El Capulin installation.

Mexico’s 35,000 or so federal police have an increasing role in the fight against the crime gangs that wrack parts of Mexico, the main conduit for cocaine from South America to the United States.

Better equipped than municipal and state police, the federal police have been afflicted by corruption, underscored by a shootout in the Mexico City airport June 25 when several federal officers shot and killed three co-workers mounting a drug sting, then fled underground.

“You have different pieces of the state working for different crime groups,” said Edgardo Buscaglia, a security expert and senior scholar at Columbia University. “I’m very doubtful that this was just an accident.”

Buscaglia said the lack of clarity of the role of the wounded Americans draws attention to the widening role of U.S. personnel in Mexico’s battle against narcotics gangs.

Fred Burton, vice president for intelligence at the Austin, Texas, analysis firm Strategic Forecasting, tweeted that he “wouldn’t be surprised if the victims were intelligence agents.”

Friday’s attack marked the third time U.S. personnel had come under fire since President Felipe Calderon came to office in late 2006 and deployed the military against crime gangs.

In February 2011, a vehicle carrying two U.S. customs agents came under fire near San Luis Potosi, an industrial city in central Mexico. Gunmen killed one of the agents, Jaime Zapata, and wounded the other, Victor Avila Jr.

Three people connected to the U.S. Consulate in Ciudad Juarez were killed in two drive-by shootings in March 2010.

Sylvia Longmire, a security analyst and author of “Cartel: The Coming Invasion of Mexico’s Drug Wars,” said the latest attack draws new emphasis to the willingness of gangsters to fire on U.S. government employees.

"Criminal groups in Mexico seem to care less and less about avoiding confrontations with US law enforcement agents,” she wrote in an email, adding that U.S. officials have not worked hard enough to bring culprits to justice for previous attacks.

“To date, no one has been indicted for the murder of Agent Zapata, and I’ll make the cynical assumption that no one will be indicted for this attack, either," she said.