El Paso, Texas » When Mexican drug traffickers need someone killed or kidnapped, or drugs distributed in the United States, they increasingly call on American subcontractors -- U.S.-based prison gangs that run criminal enterprises from behind bars, sometimes even from solitary confinement.

Prison gangs have long controlled armies of street toughs on the outside. But in interviews with The Associated Press, authorities say the gangs' activity has expanded beyond street-level drug sales to establish a business alliance with Mexican cartels.

"They'll do the dirty work that, say, the cartels, they don't want to do" in the United States. "They don't want to get involved," said a former member of Barrio Azteca, a U.S. prison gang tied to Mexico's Juarez cartel.

The partnership benefits both sides: The gangs give drug traffickers a large pool of experienced criminals and established distribution networks in the U.S. And the cartels provide the prison gangs with discounted drugs and the logistical support of top criminal organizations. To carry on their gang activity, imprisoned gang members resort to elaborate subterfuge: using sign language, sending letters through third parties, enlisting corrupt prison officials, holding conference calls using contraband cell phones. Some even conduct business in an ancient Aztec language to foil censors.

FBI special agent Samantha Mikeska spent nearly a decade

investigating the Barrio Azteca. Last year, three leaders got life sentences, but she questioned the real value of sending them back to prison.

"I think I've made them stronger," she said.

The latest annual National Drug Threat Assessment, released in February by the Justice Department, said prison gangs were operating in all 50 states and were increasing their influence over drug trafficking along the U.S.-Mexico border. Federal authorities have documented numerous links between most of the major U.S. prison gangs and Mexican drug trafficking organizations.

For instance, federal prosecutors in San Diego charged 36 people last year in a racketeering case that connected California's infamous Mexican Mafia prison gang to the Arellano Felix drug trafficking organization in Tijuana. Gang members and associates allegedly worked in drug-trafficking, kidnapping and attempted murder for the Mexican cartel.

Baldemar Rivera ran a large Texas' prison gang called Raza Unida for years while confined in isolation, which is common for gang members.

Rivera, who is known as "the Professor" and perpetuates the name with wire-rimmed glasses and calm demeanor, said he used sign language to discuss business with a subordinate who visited him in prison. When Rivera needed to communicate with gang members in other Texas prisons, he turned to his captains -- who were also imprisoned -- to write to the men.

"Within three or four days, everything was known," Rivera, 50, said recently from a medium-security facility near Cuero, where he is serving a 60-year murder sentence. He says he left gang life a decade ago after finishing the state's gang-renouncement program.

Rivera was running Raza Unida in the 1990s, when prisoners often used the mail to communicate with each other and the outside world. Now they have cell phones. Authorities confiscated 1,200 phones from Texas prisons last year.

Texas inmates can no longer mail letters directly to each other, but they now use third parties. For instance, a letter sent to a girlfriend is immediately readdressed and mailed to another gang member in prison.

Inmates also hold conference calls arranged by friends on the outside. To get around mail censors, some even communicate in Nahuatl, the language of the ancient Aztecs, which is still spoken by about 1.4 million people in parts of Mexico.

Gang members learn the language from books and use it mostly in written communications. Some leaders adopt Aztec names in Nahuatl.

The use of the language "is to honor their heritage but most importantly to conceal their messages to law enforcement," said FBI special agent Armando Ramos.

Sometimes they get help from corrupt court or prison employees. A woman who worked in the federal public defender's office was convicted in El Paso of serving as a bridge between incarcerated gang members and cohorts on the outside.

In at least two recent federal cases, gang members testified about how money from the gang's outside enterprises -- extortion, drug sales and other illicit activity -- was routinely funneled to gang members' commissary accounts in state and federal prisons.

In a case against Texas' Mexican Mafia prison gang last year, an FBI agent testified that the gang collected at least $8,000 per week and sometimes as much as $40,000, just in San Antonio. The proceeds were sent to gang leaders in prison, where they could spend it on food, stationery and other personal items, or they could send it to family members on the outside.

"Commissary money, all that, it just comes in," said the former member of Barrio Azteca, who left the gang and is serving a 40-year murder sentence in Texas. He spoke on condition of anonymity because he feared for his safety in prison and after his release.

"Drugs -- you don't even got to ask for that, it just flows in towards you. Visitations from girls that mess with the same people, with the group that you're in."

The contraband is smuggled in by corrupt guards, lawyers and visitors, and paid for with the revenue from the streets -- a cell phone can cost $2,000. The goods are dropped at pre-arranged locations where prisoners can retrieve them while on work detail. Once 60 phones were found in an air compressor delivered for a Texas prison's workshop.