''It is known that these are drug traffickers using military uniforms, and they were not even regulation military uniforms," Mexican presidential spokesman Ruben Aguilar told reporters.
A US law enforcement official said the FBI and other agencies found no evidence the uniformed men were Mexican soldiers. The official spoke on condition of anonymity because of the sensitivity of the matter.
Both countries said they were investigating the case, which comes at a time of rising anger over border security, with Washington considering extending a wall along its 2,000-mile-long frontier with Mexico -- an idea Mexicans bitterly resent.
''We have communicated at the diplomatic level with the government of Mexico on the matter and requested that they investigate the matter -- and that US authorities are already investigating the incident," State Department spokesman Sean McCormack said in Washington.
The Mexican government also cited its longstanding policy that its soldiers must stay away from the border unless they have special authorization.
In Mexico, kidnappers and drug smugglers regularly wear police gear. Caps, vests, and T-shirts bearing official-looking logos for Mexico's federal police force are sold at street stands.
Mexico has struggled to remove corrupt law enforcers and keep security equipment out of the wrong hands. But police and soldiers have been arrested and charged with carrying out drug operations and even kidnappings.
