By Ioan Grillo
TRES MARIAS, Mexico, Aug 29 (Reuters) - Two U.S. officials shot and wounded by Mexican federal police just south of the capital were CIA officers, security sources say, and the attack could badly hurt U.S.-Mexico cooperation in a war against drug cartels if found to be a deliberate ambush.
The pair of experienced officers were on their way to a Mexican Marine base on Friday, working with local authorities on a training mission, when federal police riddled their armored van bearing diplomatic plates with bullets.
The men, traveling with a Mexican Marine captain, were wounded and taken to a hospital for treatment, though their injuries were not life threatening. Television footage showed the vehicle strafed with bullet marks, its tires and rear window shot out.
A dozen federal police officers detained and questioned over the attack have been ordered held in custody for 40 days, and in initial statements to federal prosecutors they claimed they confused the Americans for criminals.
However, eyewitnesses at a bend in the road outside the small town of Tres Marias told Reuters the gunmen were dressed in plain clothes and carried rifles. They said they pursued the Americans firing from unmarked cars trying to box them in and on foot -- a classic style of gangland hits in Mexico.
"We had no idea at all they were police. They looked like criminals," said one woman who witnessed the incident but asked not to be named for fear of repercussions.
Witnesses said the CIA driver made impressive evasive maneuvers which likely saved their lives, and they believe they heard hundreds of bullets fired, estimating the incident lasted around six minutes.
One media report said shell casings from AK-47s, which are not used by Mexican police and are a weapon of choice for drug cartel members, were found at the scene, which could suggest the police were working in tandem with gangs.
Tres Marias is close to the city of Cuernavaca, a weekend retreat that has been badly hit by drug violence in recent years. In 2009, Mexican Marines shot dead leading cartel boss Arturo Beltran Leyva, alias "The Beard," in Cuernavaca based on information from the U.S. Drug Enforcement Administration.
U.S. and Mexican officials are probing last week's incident to ascertain whether it was a mix-up, or more sinister.
A Mexican official close to the investigation who asked not be identified said the CIA officers were within a few miles of the Marine base when they were shot by the men in civilian clothes.
The CIA declined to comment on the incident.
One U.S. official familiar with inquiries into the incident said that there are a "whole lot of unanswered questions" and that future dealings between the United States and Mexico could be seriously affected if the United States concludes that a major cover-up is going on.
Mexico's police services have been plagued by cases of corruption and officers working for hire for cartels in recent years, amid a surge in violence that poses a major challenge for incoming President Enrique Pena Nieto.
Roadside shootings have been a feature of the violence linked to drug gangs that has overshadowed President Felipe Calderon's six years in office. Gangs have been known to set up fake military checkpoints to ambush rivals.
Last year, two U.S. Immigration and Customs Enforcement agents were shot by hitmen on a major Mexican highway. One of the agents died.
Calderon promised on Tuesday to get to the bottom of how the pair were shot by federal police officers.
Speaking alongside the U.S. ambassador in Mexico, Calderon said the shooting incident should not hinder bilateral efforts to fight Mexico's violent drug cartels.
"We can't allow these things to happen, whether it is because of negligence, lack of training, lack of trust or complicity," Calderon said.
Major U.S. agencies including the CIA, FBI and Drug Enforcement Administration operate out of the embassy in Mexico City.
Under the "Merida Initiative" which began in 2008, U.S. agents have trained Mexican police and soldiers to help them with Calderon's war on traffickers. Washington has also supplied Mexico with equipment including Black Hawk helicopters and surveillance gear.
Much of the training and hardware has gone to the Marines, an elite force that operates out of Mexico's Navy Ministry and has captured or killed several major drug traffickers. However, the United States has also trained the federal police.
During Calderon's six-year offensive against cartels, there have been more than 55,000 drug-related murders. More than 3,000 police and soldiers have died, although many were involved with the gangs.