The sound of gunfire and convoys of soldiers become familiar south of the border in Ciudad Mier.

Many residents from the Tamaulipas border town of 6,000 people have already fled to neighboring cities on both side of the border due to violence between drug trafficking rivals.

Mexican officials are sending more troops to the troubled border city and surrounding communities.

But drug cartel expert Gary Hale told Action 4 News that the fighting between Gulf Cartel and their former allies the Zetas is expected to continue.

Hale said things may get worse before they get better.

"Mier and Camargo have traditionally been strongholds of the Gulf Cartel,” he said. “The Zetas have been, since the split in early 2010, trying to recover that territory or usurp it away from the Gulf Cartel."

Hale, the former head of the U.S. Drug Enforcement Administration’s regional office in Houston, now heads Grupo Savant.

The Katy,Texas-based consulting company specializes with law enforcement issues and cross-border intelligence.

Hale says the recent death of Gulf Cartel leader Antonio Ezequiel Cardenas-Guillen at the hands of Mexican marines in Matamoros created an unstable situation.

The drug trafficking expert said Jorge Eduardo Costilla-Sanchez, known as "El Coss,” has taken the reigns of the Gulf Cartel.

But Hale said the Gulf Cartel may be facing more enemies than its former allies the Zetas.

"Prior to this decade, all of that territory belonged to the Sinaloa Cartel and they have always wanted to recover this territory,” Hale said. “This gives them an opportunity."

Hale said there are no “good guys” in the drug war but if the Zetas end up in control, there could be other consequences.

"Extortion of businesses...kidnapping for ransom...things of that nature…I would expect that they return, they the Zetas, would return to those strong arm tactics of the past,” he said.

Hale said almost all of the casualties in the drug war have been criminal versus other criminals but some law enforcement officials involved in the drug trade have also been targets.

The drug cartel expert said there have been very few innocent civilians killed in the crossfire.

But Hale warned Rio Grande Valley residents that the best way to stay safe is not to cross into Mexico unless absolutely necessary.