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

The Mexican left-wing opposition Democratic Revolution Party said it would summon government officials to a Senate hearing in order to clarify the murky role of the US Central Intelligence Agency in Mexico.

"We will ask for a hearing with the public security minister, the foreign minister and the navy to find out what CIA agents are doing in Mexico and why they are fighting each other," Senator Mario Delgo told MVS radio.

Washington works closely with President Felipe Calderon's government against drug smuggling under the $1.6 billion Merida Initiative, providing training for law enforcement officials and equipment, including Black Hawk helicopters.

After days of feverish speculation about who the wounded Americans were working for, The New York Times reported Wednesday that the pair were employed by the CIA as part of an anti-drug task force.

The Mexican foreign ministry and the US embassy had no immediate comment on the Times report.

A US State Department spokesman would only say Tuesday that the two were US government employees working 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 armoured US embassy vehicle was fired upon by federal police, officials said.

Authorities are holding 12 police officers in connection with the shooting as prosecutors mull charges against them.

US officials told the Times on condition of anonymity there was no evidence so far the Americans, who have not been identified, were targeted because of their affiliation.

Mexico City Mayor Marcelo Ebrard, a member of the Democratic Revolutionary Party, had already raised questions about the CIA's presence in Mexico on Tuesday.

"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," Ebrard said.

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

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.

Analysts say the number of US security officials in Mexico has soared since Calderon launched an anti-drug offensive in 2006. More than 50,000 people have died since Mexican troops were deployed against the cartels.

But Calderon has refused to disclose 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," Edgardo Buscaglia, a security expert and senior research scholar at New York's Columbia University, told AFP.

Americans have shed blood before in Mexico's drug war. A US Immigration and Customs Enforcement (ICE) agent was killed and another wounded last year when Zetas cartel gunmen fired on their car between Mexico City and Monterrey.

While the US Drug Enforcement Administration (DEA) used to be the dominant force, the CIA, the US Defense Department and other American agencies have increased their presence, Buscaglia said.

"The expansion of the US presence within Mexican soil is unprecedented," he said. "We are reaching levels — not in terms of soldiers but in terms of American intelligence — that are close to Afghanistan."