The strength of the country's drug cartels inside prisons was evident in the New Year's Eve killing of Arturo Guzman Loera, brother of Sinaloa drug trafficker Joaquin "El Chapo" Guzman, inside the maximum-security La Palma penitentiary near Mexico City.
Their reach was underscored further this month when the Mexican attorney general revealed that a member of Fox's staff had been arrested after allegedly feeding information about the president's travel plans and whereabouts to an unnamed drug smuggling gang.
Revelations that followed the Mexican army's occupation of La Palma prison reinforced the impression that the inmates were more in charge than the government. Evidence found in the La Palma raid indicated that rival drug lords Benjamin Arellano Felix of the Tijuana cartel and Osiel Cardenas of the Gulf cartel had formed an alliance and were running their operations from their cells.
The government's subsequent efforts to regain control of the prison system, including searches of Matamoros and other federal prisons and transfers of some traffickers to different jails, unleashed a new round of violence, prompting the State Department warning.
Among the recent incidents in Tamaulipas state:
* On Jan. 17, the former mayor of Soto la Marina and his two sons were found dead, their bodies dumped alongside a highway.
* On Jan. 15, 20 fishermen from the coast a few miles east of Matamoros were kidnapped and beaten by an armed band of suspected traffickers who accused them of stealing a shipment of cocaine. They were freed after a day.
* On Jan. 8, Reynosa policeman Alan Gerardo Mata and his uncle were found dead on the highway to Monterrey. A note was pinned on Mata's pants saying the killing was a message to "El Chapo and those who wanted to help him."
* On Jan. 7, an armed group entered a downtown hotel two blocks from Matamoros city hall and briefly held 40 guests hostage while they searched for a rival. Some of the guests were beaten.
* Also on Jan. 7, the former mayor of the town of Diaz Ordaz disappeared; he hasn't been seen since.
* On Jan. 3, the security advisor to the mayor of Reynosa was found dead alongside the highway to Monterrey.
In addition, about 25 U.S. citizens have been kidnapped in Tamaulipas state in recent months, although mostly from the city of Nuevo Laredo and none in Matamoros.
News of the prison killings, as with all the outrages perpetrated by Mexican drug traffickers, travels fast across the border, said Sergio Lopez, owner of a trucking firm in Brownsville. He said many of his friends and business associates were reluctant to cross the border.
Julio Cesar Almanza, president of Matamoros' chamber of commerce and tourism, said that fact was driven home when he recently got a call from a Brownsville judge and others who were planning to attend a wedding in Matamoros.
"They said they want a police escort after they cross. That really made me realize how this [wave of violence] is affecting us," Almanza said. "But bad things only happen to bad people here. Nothing happens to those who do the right thing." Almanza got them an escort.
Maybe the judge had read the Matamoros newspapers too often. A front page of the daily El Bravo last week splashed a photo of the latest victim of the violence, a man found shot to death execution-style along with his wife in their rental home here.
Next to the photo was a story headlined "Defamatory Campaign Fails," quoting a local federal official as saying that the U.S. travel advisory was "exaggerated."