CIA operatives? US Drug Enforcement Agency agents? US defense contractors? Speculation is rife in Mexico over the identity of two US government employees whose car was shot up by federal police.

The US and Mexican governments have said little about the nature of their work since last week's shooting, a silence that has put a spotlight on the growing but often secretive US role in Mexico's brutal drug war.

"The expansion of the US presence within Mexican soil is unprecedented," Edgardo Buscaglia, a security expert and senior research scholar at New York's Columbia University, told AFP.

"We are reaching levels -- not in terms of soldiers but in terms of American intelligence -- that are close to Afghanistan," he said.

He added that, in addition to the DEA, the CIA, Pentagon and other US agencies now work in Mexico.

The US State Department has only disclosed that the two employees wounded in Friday's shooting worked on "law enforcement cooperation." The pair were repatriated to the United States over the weekend.

The two were driving with a Mexican navy captain to a military training facility south of Mexico City on Friday when their armored US embassy vehicle was fired upon by federal police, according to Mexican officials.

While 12 police officers have been detained as prosecutors investigate the shooting, some Mexican media and politicians have focused on the two US government employees and the role of US security agents in Mexico.

The magazine Proceso, citing anonymous sources, said the two "ghosts" were DEA agents participating in a mission to track down Hector Beltran Leyva, a druglord whose brother was killed by Mexican marines in December 2009.

Others have speculated that they worked for the CIA or were American contractors here to help train Mexican security forces.

"The Mexican government must give a complete report on what the CIA is doing here, with whom it is working and what is the extent of its work," said Mexico City Mayor Marcelo Ebrard, a member of a left-wing opposition party.

"Everything is in the dark," he told reporters, adding that Mexico traditionally does not authorize CIA operations.

Americans have shed blood before in Mexico's drug war, which has left more than 50,000 people dead since President Felipe Calderon deployed soldiers to combat the powerful, feuding cartels.

Two US Immigration and Customs Enforcement (ICE) agents were shot by Zetas cartel gunmen while driving between Mexico City and the northern city of Monterrey in February 2011. One of them, Jaime Zapata, died in the attack.

The United States cooperates closely with Mexico to combat drug smuggling under the $1.6 billion Merida Initiative, which provides training for Mexican law enforcement officials and equipment, including Black Hawk helicopters.

America's involvement in the affairs of Latin American nations is always a sensitive issue, given its support to military regimes during the Cold War.

"The Mexican government has historically mistrusted the United States due to its tendency to intervene in every Latin American country," Javier Oliva, a political science professor at the National Autonomous University of Mexico (UNAM), told AFP.

Calderon's government has been forced in the past to defend the presence of US agents or the use of US drones over Mexican territory in the fight against drug cartels.

But it has refused to publicly release the number of US law enforcement agents working in Mexico. Under Mexican law, foreign agents or soldiers are forbidden from taking part in operations or carrying weapons in the country.

"Of course many of these operations are taking place, and of course they are bypassing the legal framework in doing so," said Buscaglia, the Columbia University scholar.