ENLARGE

A U.S. Marshals Service inspector, injured in a firefight in July between the Mexican military and drug cartel suspects, on a hospital roof in Culiacán, according to people familiar with the incident. AVEL AVILEZ/EL DEBATE

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.

ENLARGE

The Marshals Service referred questions to the Justice Department, of which it is a part.

A Justice Department spokeswoman said, “The U.S. Marshals have an important—and sometimes dangerous—mission of capturing fugitives and facilitating extraditions in the United States and around the world.”

One U.S. official said the missions are approved at a high level of the Mexican government.

The Mexican embassy in Washington denied that Mexico’s government gave U.S. agencies permission to go on armed raids. “Members of foreign law enforcement agencies or foreign military, including those from the U.S., are not authorized to carry weapons within the Mexican territory, and none of them are authorized either to participate in any raids or other armed law enforcement operations,’’ said a spokesman, Ariel Moutsatsos-Morales.

The missions represent a new example of risks the Justice Department is taking in pursuing Mexican cartels. A 2010 program called Fast & Furious, in which the U.S. allowed the purchase of weapons by suspected “straw” buyers in an effort to track them to cartel figures, led to a scandal when one of the guns was linked to the killing of a U.S. Border Patrol agent. In the aftermath, the head of the Bureau of Alcohol, Tobacco, Firearms and Explosives was forced out and others were punished or resigned. Attorney General Eric Holder has said he wasn’t aware of the plan until later.

The new disclosures are likely to strike a raw nerve in Mexico, where the presence of armed U.S. agents on its soil has long been a contentious issue. In Washington, the shootout in July sent shock waves through the select circle of law-enforcement officials aware of the operation, people familiar with the matter said.

Generally, U.S. law-enforcement agents overseas are prohibited by local laws from carrying weapons, and they have no arrest powers outside the U.S.

The State Department declined to discuss law-enforcement cooperation between the U.S. and Mexico.

The Marshals Service operations in Mexico are carried out by a small group sent for short, specific missions. The goal is to help Mexico find and capture high-value cartel targets.

One operation yielded a great success: The capture of cartel boss Joaquin Guzman Loera, known as “ El Chapo, ” earlier this year. It is unclear whether U.S. Marshals personnel were disguised as Mexican military men on the day he was caught.

Sometimes the Marshals Service targets a person Mexico would like to apprehend but who isn’t wanted by U.S. authorities, the people familiar with the work said.

Marshals personnel on the ground dress in local military garb to avoid standing out and are given weapons to defend themselves. When a mission goes badly, as on July 11, one of the people familiar with the work added, “it can turn into a flat-out kill mission.”

Some of them worry that U.S. personnel could be charged with a crime and jailed in Mexico if a mission went particularly badly or if they ran afoul of the wrong local official.

The Marshals Service works closely with the Mexican Marines because the U.S. agency has expertise at finding fugitives, in part through technology that detects cellphone signals and other digital signatures. That includes airplane flights operated by the agency carrying sophisticated devices that mimic cellphone towers, as reported last week by The Wall Street Journal. That technology works better with a ground presence.

Responding to the Journal article last week, a Justice Department official said that “any investigative techniques which the Marshals Service uses are deployed…only in furtherance of ordinary law enforcement operations, such as the apprehension of wanted individuals.”

The people familiar with the matter described the Marshals Service as a police agency affected by mission creep. More than five years ago, the Service flew small planes along the border to detect cell signals and locate suspects inside Mexico. About four years ago the flights crossed deep into Mexican airspace, the people said.

They added that, more recently, some flights have been conducted in Guatemala.

ENLARGE

U.S. Marshals Service director Stacia Hylton, shown in 2012, emailed colleagues after a July firefight in Mexico’s Sinaloa state saying that a Marshals inspector injured in the incident was in stable condition and recovering. GETTY IMAGES

A spokeswoman for the Guatemalan embassy in Washington had no immediate comment.

The plan for the July mission in Sinaloa, hundreds of miles from the U.S. border, was typical, said those familiar with it—but quickly went wrong.

Members of the FBI, DEA and Marshals Service met with a group of Mexican Marines in preparation for a Friday raid. The goal was to apprehend a senior member of Los Mazatlecos, a gang of enforcers with ties to the Beltran Leyva drug cartel.

A handful of Marshals specialists dressed themselves as Mexican Marines and took Marines weapons. As they and the Mexican Marines set off on foot, a small plane flown by a U.S. Marshals employee kept an eye on the target site, advising colleagues on the ground who in turn guided those on foot. DEA and FBI personnel remained a mile or so away in an armored vehicle, observing and advising.

The men walked through a field toward the site. As they approached a line of bushes, hidden gunmen opened fire. A U.S. Marshals employee with the rank of inspector was shot in the arm and fell. A Mexican Marine rushed to carry him to safety and was also hit. Then another shot struck the American in the torso.

After the firefight, the wounded American was airlifted to a hospital in Culiacán, where he was kept under guard until he could be moved to a hospital in San Antonio.

U.S. officials scrambled to keep the incident quiet, people familiar with the operation said. One senior U.S. official in Mexico told the other law-enforcement personnel to “forget they were here,” those familiar with the matter said. The official was told that would be difficult because one person had already notified superiors in Washington of the shooting.

The U.S. Marshals pilot who provided reconnaissance was told by superiors to leave Mexico in the middle of the night, people familiar with the operation said.

Stacia Hylton, director of the Marshals Service, sent colleagues an email days after the firefight saying the inspector “is in stable condition and recovering at a hospital with his family in the United States,” according to a copy reviewed by the Journal.

She added: “The laser-focus in which you accomplish the mission in your area of expertise is valued tremendously from our law enforcement partners, just as it is throughout our investigative programs domestically.”

The Marshals Service hasn’t said anything publicly about the inspector’s shooting. Spokesmen for the FBI and DEA declined to comment.

The July clash hasn’t altered the agency’s position on such raids, according to the people familiar with it. In recent weeks, the Marshals Service has been planning another covert mission in Mexico, they said.

Write to Devlin Barrett at devlin.barrett@wsj.com