MEXICO CITY -- Mexican marines detained a man believed to be the leader of the Gulf drug cartel in the northern state of Tamaulipas, across the border from Texas, the navy announced late Wednesday.

The navy said in a brief statement that the man identified himself as Gulf cartel boss Jorge Eduardo Costilla Sanchez. The navy gave no other details, saying it would provide more information Thursday.

If confirmed, the arrest of Costilla would be another blow to the Matamoros-based Gulf Cartel, which has been weakened by its struggle to keep the rival Zetas cartel from taking over all of its territory since the two split in early 2010 and began a bloody turf war.

A verified capture also would be a significant victory for the marines, who were embarrassed in June after announcing they had nabbed the son of Mexico's top fugitive drug lord.

It turned out the man wasn't the son of Joaquin "El Chapo" Guzman, but rather Felix Beltran Leon, 23, a stocky, baby-faced suspect whose family said he was the father of a toddler and worked with his mother-in-law at a used car dealership. He remains in custody, authorities say, because guns and money were found when he was arrested.

Costilla was born in Matamoros, across the border from Brownsville, Texas, and worked for several years as a local police officer. He joined the Gulf Cartel in the 1990s and became a lieutenant for then-leader Osiel Cardenas Guillen.

After Cardenas Guillen was arrested in 2003, Costilla joined the capo's brother Ezequiel in running the cartel, which smuggles and distributes tons of cocaine, methamphetamine, heroin and marijuana into the United States.

Osiel Cardenas Guillen was extradited to the United States in 2007 and sentenced to 25 years in prison by a Texas court in February 2010.

Ezequiel was the cartel's figurehead until he was killed in November 2010 in a shootout with Mexican marines in Matamoros, but authorities believe Costilla controlled the cartel's daily drug trafficking activities.

The announcement of the arrest comes just more than a week after the navy said it had detained another brother of Osiel, Mario Cardenas Guillen, in the Gulf Coast city of Altamira.

The navy said when announcing Mario's arrest the cartel had apparently divided into two wings after Ezequiel's death.

Arrests of high-ranking cartel leaders often lead to detentions of others, some because officials find intelligence with the suspects, others because detainees swiftly turn against their former comrades and provide information that leads to their arrest.

Nicknamed "El Coss," Costilla was a somber capo who kept a low profile. Only two photographs of the round-faced, mustached drug trafficker were ever made public.

He was linked to the August 2004 beating death of Matamoros newspaper columnist Francisco Arratia Saldierna, who reported on drug trafficking and organized crime.

Costilla, 41, was indicted on drug-trafficking charges in the U.S., where authorities offered a $5 million reward for information leading to his arrest. Mexican authorities offered a $2 million reward and had him on their list of the nation's most-wanted drug traffickers.

Costilla was also indicted for threatening U.S. law enforcement officials in November 1999.

In that incident, a Drug Enforcement Administration agent and an FBI agent, both assigned to the U.S. consulate in Monterrey, had been followed by a car through Matamoros until a truck cut them off.

They were quickly surrounded by about a dozen heavily armed men, allegedly including Costilla and Osiel Cardenas Guillen, who threatened to kill them. The agents eventually persuaded the gunmen to let them go.

At that time, the Gulf cartel firmly controlled northeastern Mexico.

The balance of power began shifting after Osiel Cardenas Guillen was arrested in 2003. The Zetas gang, which was founded by former Mexican military special forces and worked for the Gulf Cartel as assassins, pushed for independence and spread south.

The two groups split definitively in 2010 over the killing of a Zeta member in the city of Reynosa, across from McAllen, Texas, in January 2010. Costilla reportedly ordered the slaying.

The resulting fighting between the two gangs transformed northeastern Mexico into a war zone with daily shootouts and other gruesome violence that included decapitations and corpses hanging from overpass bridges.

Associated Press