A regional security chief in the Mexican border state of Tamaulipas was shot to death along with his wife, authorities said Monday.

It was the latest round of violence in Tamaulipas, across the border from Texas, and about 2,000 residents gathered in the state capital of Ciudad Victoria to protest the killings and disappearances.

The dead man, Ricardo Nino Villarreal, was security chief for the area around Nuevo Laredo, across the border from Laredo, Texas. The area is considered the turf of the vicious Zetas drug cartel, which has been battling the Gulf cartel.

Nino Villarreal was a former general and one of several ex-military officers designated as regional commanders as part of a plan announced in May to stem a wave of violence in Tamaulipas.

The state government said the killing took place Saturday on a roadway in neighboring Nuevo Leon state. However, it wasn't until Sunday that passers-by noticed the couple's car and alerted police.

The car was a compact model and the couple was travelling on a non-toll road that links Nuevo Laredo to the industrial hub of Monterrey. The state government said armed assailants ambushed them.

Despite the government plan for beefed-up security, Tamaulipas has seen continued violence.

On Monday, residents clad in white and lead by white-robed priests marched through Ciudad Victoria, carrying photos of people who have disappeared due to kidnappings and drug gang violence.

The protesters estimated that at least 750 people have been killed or disappeared in the area over the last four years.

Roman Catholic Bishop Antonio Gonzalez Sanchez questioned whether any progress was being made, despite accords being signed to combat crime.

"What good have those signatures done? I am just asking that question," Gonzalez Sanchez said.

Last week, investigators found the bodies of three U.S. citizens and another man who had been shot to death near the city of Matamoros, Tamaulipas.

The Americans had been reported missing on a visit to Mexico two weeks before. The fourth body is believed to be the Mexican boyfriend of one of the slain Americans.

Tamaulipas officials have said they were investigating a possible police connection to the deaths. The family of the Americans has said witnesses reported they were taken away from a restaurant by armed men who identified themselves as part of the "Hercules" tactical security unit in Matamoros.

Each of the four dead had been shot in the head and the bodies were burned, most likely from lying in the hot sun for so long,

And in mid-October, the nearby city of Reynosa was shocked by the kidnapping of Dr. Maria del Rosario Fuentes Rubio, an activist who used Twitter and Facebook accounts to warn residents about drug gang shootouts.

Fuentes Rubio's relatives filed a complaint saying she was kidnapped by gunmen outside a business in Reynosa. The Tamaulipas prosecutors' office said it was investigating reports she had been killed, an apparent reference to images that were posted on her hacked Twitter account after she disappeared.

Before the hacked account was taken down, someone using Fuentes Rubio's cellphone posted a photo of her apparently lifeless, bloody face and the words "today my life has reached its end."