Mexico has begun a formal investigation into whether terrorists plan to use Mexican flights to launch an international attack, the top organized crime investigator in the country said Tuesday.

The United States, Canada and Interpol told Mexico that officials suspect terrorists might be using Mexican soil to plan an attack, Deputy Attorney General Jose Vasconcelos (search) told reporters.

Officials canceled two Aeromexico (search) flights from Mexico City to Los Angeles last week because of terrorism concerns, but Mexican authorities found no evidence of an attack plot.

A formal investigation was opened in the first days of January "because of the exchange of information about the probability that there could be some attack or terrorist act against some airport installation in Mexico," Vasconcelos said.

The type of investigation Vasconcelos announced is an essential step toward filing criminal charges under Mexican law. Vasconcelos wouldn't say if authorities planned to charge anyone for conspiring to commit acts of terrorism, however.

He said investigators did not have specific information about whether there was going to be an attack against an airplane. "It is just the probability of a terrorist act in flights that are leaving Mexico City," he said.

On Monday, Mexico said it had increased security measures for international flights two weeks before the canceled flights, and had placed armed Mexican agents on some flights bound for the United States. The measures have delayed many flights by at least 30 minutes and sometimes for hours.

Unarmed U.S. agents also have been seen at the airport, apparently checking passenger lists and sharing information with Mexican officials.

Miguel Angel de la Torre, chief of the police Special Operations Group, said Tuesday that about a dozen patrol cars have been circling the airport watching for suspicious activity.

Officials say that hundreds of police and security agents from a battery of agencies are involved in the security operation in Mexico City.