Cardenas Guillen, also known as "Tony Tormenta" or "Tony the Storm," is the brother of imprisoned former leader Osiel Cardenas Guillen and is believed to have run the powerful cartel along with Jorge Eduardo Costilla Sanchez.
He is the latest in a growing number of high-profile cartel leaders who have been captured or killed by the armed forces President Felipe Calderon has stationed throughout the country to battle drug traffickers.
The day of clashes across the border from Brownsville, Texas, also claimed the lives of three gunmen and two marines, said Alejandro Poire, Calderon's security spokesman. A soldier and a local reporter were also killed in the clashes, the Mexican Defense Department said in a news release.
Gunfire first broke out about 11 a.m. at an upscale residential area in Matamoros and shootouts ensued throughout the city after that for about eight hours, said a resident who asked not to be identified for fear of reprisal.
A video posted on YouTube shows a string of SUVs and pickup trucks racing through a street while continuous shooting is heard in the background. Men wearing ski masks get out of a car and use it to block the street.
Cardenas Guillen, 48, had been indicted in the United States on drug-trafficking charges and U.S. authorities had offered a $5 million reward for information leading to his arrest. Mexican authorities offered a $2 million reward and had him on a list of the nation's most wanted drug traffickers.
His death is a blow to Mexico's second-most powerful cartel and a major boost to Calderon's war on drug cartels. U.S. authorities considered him a key trafficker of cocaine and marijuana north.
"Today, we have taken another meaningful step toward the dismantling of criminal groups that do so much damage to our country," Poire said.
The deceased trafficker's brother Osiel Cardenas Guillen led the Gulf cartel until his arrest by Mexican authorities in 2003. Osiel Cardenas Guillen was extradited to the United States in 2007 and sentenced to 25 years in prison by a Texas court in February.
Northeastern Mexico, an area once controlled by the Gulf cartel, has seen an increase in violence due to a turf battle between the cartel and the Zetas, a drug gang made up of former Mexican special-forces soldiers. The violence has included broad-daylight shootouts and dozens of beheaded corpses dumped in public areas.
The Matamoros newspaper El Expreso said on its website that reporter Carlos Guajardo was covering the shootout when he was struck. Local news media reported Guajardo was leaving the area of the clash when his car was hit by gunfire more than 20 times.
The Committee to Protect Journalists said in a recent report that at least 22 Mexican journalists have been killed since December 2006, when Calderon launched his crackdown on the cartels. Nationwide, more than 28,000 people have been killed in drug violence.
The death of Cardenas Guillen follows a string of killings or captures of suspected drug lords.
Arturo Beltran Leyva, leader of the Beltran Leyva cartel, died in a raid outside Mexico City on Dec. 16, 2009. Mexican soldiers killed the Sinaloa cartel's No. 3 capo, Ignacio "Nacho" Coronel, on July 29 of this year. On Aug. 30, federal police announced the capture of Edgar Valdez Villarreal, alias "La Barbie," and on Sept. 12 Mexican marines captured Sergio Villarreal Barragan, another presumed Beltran Leyva leader.
Earlier Friday, Mexican authorities announced that eight members of a drug cartel were arrested in the torture and slaying of the brother of a former state attorney general. The man had been forced to appear at gunpoint in a video saying his sister worked for a rival gang.
The body of Mario Angel Gonzalez Rodriguez was found half buried in a house under construction in Chihuahua city after one of the suspects told officials where they could find him, Federal Police Commissioner Facundo Rosas told a news conference. He said the body showed signs of torture.
The suspect said a man known as "The Vulture" ordered the group to kidnap Gonzalez, Rosas said. The men are suspected of working for the Sinaloa cartel.
Gonzalez, an attorney, was kidnapped from his office Oct. 21 -- less than three weeks after his sister Patricia Gonzalez stepped down as Chihuahua state's attorney general due to a change of governor.
Days after the kidnapping, a video was posted on YouTube showing Gonzalez handcuffed and surrounded by five masked men pointing guns at him. Prodded by an interrogator, he blamed his sister for several notorious killings in the state and said both he and she had aided "La Linea," a drug gang tied to the Juarez cartel.
Rosas said some of the men arrested had appeared in the video, which he said was shot at a safe house for the Sinaloa cartel.
Cartels have increasingly taken to releasing video clips of abducted police, officials and regular citizens admitting to crimes that aided rivals of the kidnappers. In several cases, the subject of the video has been found dead shortly afterward.
Gonzalez was attorney general during the most violent peacetime period in the history of Chihuahua state. A nearly 3-year-old turf war between the Juarez and Sinaloa drug cartels has made Chihuahua the deadliest state in Mexico, and Ciudad Juarez, on the border, one of the world's most dangerous cities.
In the video, the questioner prompted Mario Gonzalez into saying that his sister ordered several killings in Ciudad Juarez, where drug-gang violence has claimed more than 6,500 lives over the past three years. Among those, he said, was the 2008 killing of Armando Rodriguez, a crime reporter for the newspaper El Diario de Juarez.
Rosas said one of the suspects told police that Gonzalez was beaten on his feet and ankles before the video was made. The suspect also said Gonzalez was reading from a script prepared by his captors when he named state police officers and government officials supposedly in the pay of La Linea, Rosas said.
More than 28,000 Mexicans have been killed in drug-related violence since Calderon launched a national assault on organized crime in late 2006.