MEXICO CITY (AP) — Eight suspected drug gang gunmen died in a battle with Mexican soldiers in the remote mountains of northern Chihuahua state, the federal Public Safety Department said Thursday.

The department cited an internal army report saying the clash occurred near the rural town of Madera, about 145 miles (230 kilometers) south of the U.S. border.

The gunmen apparently opened fire on an army patrol, but the Defense Department did not offer any information on the attack or the identity of the attackers. The area is frequently used by gangs to produce and traffic drugs.

Also Thursday, the U.S. Treasury Department added two companies owned by daughters of drug lord Ismael “El Mayo” Zambada to the list of sanctioned companies under the Narcotics Kingpin Designation Act.

In a press statement, the U.S. Embassy in Mexico City said the two women, Maria Teresa Zambada Niebla and Midiam Patricia Zambada Niebla, served as “front persons” for their father’s illicit transactions. The companies named to the list are Arte y Diseno de Culiacan SA de CV and Autotransportes JYM SA de CV.

Any assets the businesses may have under U.S. jurisdiction are now frozen and U.S. citizens are barred from any financial or commercial dealings with them.

Zambada is wanted on drug trafficking charges issued by at least three U.S. district courts. His daughters had been identified by the Treasury Department as participants in his gang’s operations since 2007.

Also Thursday, the army command in the border state of Tamaulipas said soldiers seized two extended pickup trucks painted with Mexican army emblems and colors near the border community of Ciudad Mier.

A statement by the Eighth Military Region command said drug traffickers had painted the trucks “to disguise themselves as military personnel” and “confuse the public and cover up their illegal acts.”

The army has been the subject of numerous human rights complaints since it moved into a front-line role against drug gangs in late 2006. The military claims drug gangs have used army uniforms to discredit troops.

On Wednesday, the border city of Nuevo Laredo was practically paralyzed by late-night gunbattles in which gangs forced citizens from their cars and used the vehicles to block streets. The sound of gunfire alarmed Texans on the U.S. side of the Rio Grande.

The Nuevo Laredo city government posted messages on its Facebook page warning citizens to stay indoors as shooting erupted at several intersections in the city across from Laredo, Texas.

Frightened people on the U.S. side of the border called emergency dispatchers after hearing the gunfire, Laredo police spokesman Joe Baeza said Thursday. But he said there was no spillover violence.

“We were getting reports from people who live on the river’s edge that they could hear gunfire and explosions from the Mexico side,” Baeza said.

“We didn’t have any incidents on the American side. It’s hard for people to understand who don’t live here,” he added. “They’re not Vikings, they’re not going to invade us; it doesn’t work that way.”

Nuevo Laredo city officials said they could not immediately confirm witness reports that several gunmen were killed.

Gangs used stolen cars and buses to block several main avenues. When the violence subsided, the government urged citizens to come forward and reclaim their stolen vehicles.

Nuevo Laredo is among several northern cities under siege from a turf war between the Gulf cartel and its former enforcers, the Zetas gang of hit men. Violence has surged along the northeastern border with the United States since the two gangs split earlier this year.

In the northern state of Chihuahua, a banner appeared on a bridge threatening violence against “innocents” unless the state government fires its chief of police intelligence, Fernando Ornelas, the Diario de Juarez newspaper reported Thursday.

The banner appeared in the state capital, also called Chihuahua.

Last week, drug gangs introduced a new threat to Mexico’s drug war, detonating their first successful car bomb. The attack killed a federal police officer and two others in Ciudad Juarez, Chihuahua’s largest city.