Felipe Calderon, the Mexican president, has voiced regret for the federal police shooting of a US embassy car that wounded two US government employees.
In his first public remarks about Friday's shooting, Calderon told the US ambassador to Mexico that the attorney general's office was thoroughly investigating the incident.
"All I ask for is justice, and they [prosecutors] act according to law, and they don't treat them as criminals, because they are not criminals"
- Georgina Segobia, the sister of a detained federal police officer
"I want to express, ambassador, deep regret over the events from a few days ago," Calderon said on Tuesday, during a forum on security attended by the US envoy, Anthony Wayne.
"Whether it was negligence, lack of training, lack of trust, or complicity, this cannot be tolerated," he said. Wayne said the US and Mexican governments were "working hard" to prepare the investigation. "We will see the results," he added.
A judge on Monday placed 12 police officers under 40-day detention as prosecutors mull charges against them for the shooting, which the US embassy has described as an "ambush".
The US State Department has not identified the two government employees or the nature of their work in Mexico.
The pair were heading to a military training facility south of Mexico City with a Mexican navy captain when their sport-utility vehicle with embassy plates was barraged with bullets from four cars chasing them, according to the Mexican navy and public security ministry.
A State Department spokesperson said the two US government employees were stable enough to be evacuated to the US on Saturday and they are still receiving medical treatment in their home country.
The shooting incident took place on a highway on the southern outskirts of Mexico City close to the city of Cuernavaca, which has been ravaged by criminal gangs during the government's conflict with drug cartels.
Relatives and supporters of the detained officers gathered on Monday outside prosecutors' offices in the city of Cuernavaca, just south of Mexico City, to protest their detention.
"All I ask for is justice, and they [prosecutors] act according to law, and they don't treat them as criminals, because they are not criminals," said Georgina Segobia, the sister of a detained federal police officer.
The relatives claimed the officers were simply doing their jobs in setting up a dragnet for criminals.
The US Embassy in Mexico City said on Monday that two US government employees and a Mexican Navy captain were heading to a training facility outside the city of Cuernavaca when they were ambushed by a group of gunmen that included federal police.
The Mexican government said federal police were conducting unspecified law-enforcement activities in the rural, mountainous area known for criminal activity when they came upon the car, which attempted to flee and came under fire from gunmen in four vehicles including federal police.
Battling drug cartels
Roadside shootings have been a feature of the violence linked to drug gangs. Gangs have been known to set up fake military checkpoints to ambush rivals.
Earlier this month, all 348 federal police officers at Mexico's international airport were replaced after police there shot dead three fellow officers in an alleged drug related killing. Three officers have been charged.
Last year, two US Immigration and Customs Enforcement agents were shot by hitmen on a major Mexican highway.
One of the agents died.
Since 2008, the US government has given 243 million US dollars in equipment and 25 million US dollars in technical assistance and training to the federal police under the drug-war aid program known as the Merida Initiative.
Under Merida, the State Department says more than 4,300 federal police have completed training at Mexico's Federal Police Academy in San Luis Potosi.
Taught by law enforcement professionals from the US, Colombia, Spain, Canada, and the Czech Republic, the program includes criminal investigative techniques, evidence collection, crime scene preservation, and ethics.