MEXICO CITY -- A day after marines killed a reputed powerful drug lord, dozens of ominous banners apparently hung by rivals appeared Saturday in cities across Mexico's Gulf coast with messages gloating about his demise.

The signs, hung on pedestrian bridges and other public places but quickly taken down by authorities, reinforced fears that the death of alleged Gulf cartel leader Antonio Ezequiel Cardenas Guillen will further empower the Zetas, a gang of hit men formed more than a decade ago by renegade Mexican soldiers that has become one of Mexico's most brutal and feared drug gangs.

Former allies of the Gulf cartel, the Zetas went independent earlier this year, unleashing a turf battle along the northeastern border with the United States that has at times reached the level of all-out war.

Drug gangs used vehicles to put up roadblocks Saturday morning in Reynosa, a city across from McAllen, Texas, according to Twitter messages from the Reynosa city government warning citizens to travel with caution. It was unclear if the roadblocks were related to the death of Cardenas Guillen.

Such blockades have become a near-daily gang tactic in northeastern Mexico.

But Friday night, buses and cars were used to block roads and highways in the western city of Morelia -- stronghold of La Familia cartel -- suggesting that the tactic could be spreading across Mexico. Several vehicles were torched, and one driver was injured when assailants stole his bus to use in the blockades, according to the attorney general's office of Michoacan state.

Cardenas Guillen, known as "Tony Tormenta" or "Tony the Storm," was killed Friday during a two-hour shootout in an operation that included 150 marines, three helicopters and 17 military vehicles, the Mexican navy said in a statement. Four of his gunmen and three marines also died during the fighting in the city of Matamoros, across from Brownsville, Texas. A local reporter and a soldier were killed in related mayhem.

President Barack Obama called his Mexican counterpart, Felipe Calderon, on Saturday morning to reaffirm U.S. support for Mexico's efforts to fight the cartels, and express his condolences for the troops killed in the shootouts, according to a White House statement.

Mexican presidential security spokesman Alejandro Poire touted Cardenas Guillen's downfall as "another meaningful step toward the dismantling of criminal groups."

The Zetas, apparently, also celebrated.

The banners, which appeared in several cities of northeastern Tamaulipas state, neighboring Veracruz state, and in the resort city of Cancun on the Yucatan Peninsula, crowed: "Once again the destiny of the traitors has been demonstrated, crushing the Gulf traitors."

They were signed, "Sincerely, the Zetas Unit."

Veracruz Public Safety Secretary Sergio Lopez said authorities were not yet sure who put up the poorly written and obscure messages, and could not confirm that they were a reaction to the death of Cardenas Guillen.

Francisco Alor, the attorney general of the Caribbean state of Quintana Roo, said several banners appeared in the resort city of Cancun and he expressed concern that violence in the region could spike.

"We have to prevent any surge (in violence) that the others could generate as they move to control territory amid the power vacuum," Alor told The Associated Press, referring to the Zetas.

Drug war experts warned that the Zetas will strengthen as the Gulf cartel weakens.

"Yes, it's another major blow to the one of the most significant Mexican drug cartels," Gary Hale, who retired this year as an intelligence officer in the U.S. Drug Enforcement Administration and founded the Grupo Savant consulting firm, said of Cardenas Guillen's death.

"But it also bodes an ominous future for the war-weary civilians living in the northeastern part of Mexico," he said in an analysis sent to AP. "The ensuing chaos brought on by the loss of Gulf Cartel leadership will give operational, political and psychological strength to the Zetas."

Even before their split from the Gulf cartel, the Zetas started growing into a powerful gang in their own right. Their reach extends into Central America, where authorities have dismantled Zetas training camps, and their illegal activity ranges from drug trafficking to migrant smuggling, kidnapping and extortion.

Hale warned that the Zetas, in their push to assert dominance over northeastern Mexico, are likely to increase "their traditional organized crime activities such as extortion of businesses of all sorts, kidnappings for ransom, an increase in taxation ... for drugs crossing into the U.S. from Mexico."

The banners Saturday vowed that "the Zetas will be there" to finish off their rivals and appeared to lament that the Mexican marines "got ahead of us."

It's a chilling message if it really did come from the Zetas, an organization believed to have introduced the now-common cartel tactic of beheading rivals. Authorities have blamed the gang for the worst cartel massacre in Mexico's history: the August slaughter of 72 South and Central American migrants who apparently refused to work for the Zetas.

The banners promised revenge against "traitors ... from the school of the informant Osiel," an apparent reference to the slain trafficker's brother, Osiel Cardenas Guillen, who led the Gulf cartel until his arrest by Mexican authorities in 2003. Osiel was extradited to the United States in 2007 and sentenced to 25 years in prison by a Texas court in February.

Hale said Osiel Cardenas Guillen is rumored to have cooperated with U.S. law enforcement from prison. Several Zetas bosses were killed or arrested in the period between the capo's extradition to the U.S. and his sentencing, Hale said.

While no definitive link has been made between Osiel Cardenas' cooperation and the downfall of the Zetas leaders, the Zetas became "increasingly paranoid" and finally split from the Gulf cartel early this year, Hale said.

The downfall of Cardenas Guillen is a potential boon for the Zetas just as they are fighting off an assault from three powerful rivals. Mexican and U.S. officials say the Gulf cartel enlisted the help of the Sinaloa and La Familia gangs earlier this year in their campaign to destroy the Zetas.

"It will be even more difficult to defeat Los Zetas now because the Gulf Cartel -- part of the troika (Gulf, Sinaloa, and La Familia) ... has been weakened," said George Grayson, a Mexico expert at the College of William and Mary in Williamsburg, Virginia, who has written books about the Zetas.