MEXICO CITY, March 5 (Reuters) - A former Mexican drug cartel boss is free and back in Mexico after being released from a U.S. prison this week, a source at the Mexican Attorney General's office said on Wednesday.

Francisco Rafael Arellano Felix, 58 and the eldest of a clan of brothers who ran Mexico's Tijuana cartel, was released at the U.S. border city of El Paso, Texas, on Tuesday and crossed to Mexican soil through Ciudad Juarez.

"He does not have any pending charges in Mexico so he was freed," the source, who spoke on condition of anonymity, told Reuters.

Arellano Felix was the boss of the Tijuana cartel when he was arrested in 1993 in Mexico and sentenced to 11 years for drug possession and using illegal weapons.

He remained in prison for two extra years while authorities arranged his extradition to the United States, where he was wanted for selling cocaine to an undercover U.S. agent. He was extradited in September 2006 and pleaded guilty to the cocaine charge in June 2007 in San Diego.

His younger brothers Francisco Javier and Benjamin are behind bars in the United States and Mexico, respectively. Another brother, Ramon, was killed in a shootout with police in 2002, and a fifth, Francisco Eduardo, is at large.

The family, notorious for ruthless killings and smuggling millions of dollars of illegal narcotics into the United States, has been weakened by the loss of its top leaders but authorities say it is still doing business.

Arellano Felix's release took place two days after suspected cartel operatives fought police in a five-hour shootout in Tijuana, a crime-ridden city across the border from San Diego.

The border city has seen a spurt in violence in recent weeks as drug traffickers locked in turf wars with rival gangs react to increased police surveillance under President Felipe Calderon's army-led crackdown on drug gangs across Mexico.

Drug-related violence killed more than 2,500 people last year and at least 300 so far this year. Calderon sent thousands of troops and federal police out to drug hot spots a year ago.

On Tuesday, five youths were tortured, sprayed with bullets and dumped in an empty city lot in Tijuana in what appeared to be the latest grisly drug gang killing. (Reporting by Anahi Rama; Writing by Cyntia Barrera Diaz; Editing by Catherine Bremer and Eric Beech)

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