U.S. Justice Department personnel are disguising themselves as Mexican Marines to take part in armed raids against drug suspects in Mexico, according to people familiar with the matter, an escalation of American involvement in battling drug cartels that carries significant risk to U.S. personnel.

Both the U.S. and Mexican governments have acknowledged in the past that American law-enforcement agencies operate in Mexico providing intelligence support to Mexican military units battling the cartels. The countries have described the U.S. role as a supporting one only.

In reality, said the people familiar with the work, about four times a year the U.S. Marshals Service sends a handful of specialists into Mexico who take up local uniforms and weapons to hide their role hunting suspects, including some who aren’t on a U.S. wanted list. They said agents from the Federal Bureau of Investigation and Drug Enforcement Administration play a supporting role, in similarly small numbers.

The risks became clear on July 11, when Mexican Marines and a handful of U.S. Marshals personnel dressed as Mexican Marines were fired on as they walked through a remote field in Sinaloa state. One American was shot and wounded, and in the gunfight that followed, more than a half-dozen suspected cartel soldiers were killed, according to people familiar with the incident. It is unclear whether U.S. Marshals personnel shot anyone.

The secret missions are approved by senior U.S. Marshals executives and by leaders within the Mexican Marines, the people familiar with them said. It isn’t clear who else in either government may have given authorization.