With federal agents armed with automatic weapons standing guard on the courthouse roof, four drug lords who had been serving time in Mexican prisons were brought shackled into an American courtroom Monday to face drug and intimidation charges.

Among them was Osiel Cardenas-Guillen, believed to be one of the kingpins most responsible for violence along the Texas-Mexico border in recent years, said Karen Tandy, chief of the U.S. Drug Enforcement Administration.

"This is the first time in the history of Mexico that they have extradited to the United States what amounts to a clean sweep geographically of the cartel leadership," Tandy told reporters by teleconference from Washington.

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The Mexican government was willing to extradite Cardenas-Guillen and the other suspected drug lords because they had been able to continue their drug operations from within prison in Mexico, not because of U.S. pressure, officials said. They were flown into Houston late Friday and early Saturday.

They faced a federal magistrate under extraordinary security measures, including agents with automatic weapons perched on the roof and around the outside of the downtown federal building.

Besides Cardenas-Guillen, the group includes Hector Palma Salazar, the suspected former leader of the Sinaloa cartel, and brothers Ismael and Gilberto Higuera Guerrero, former chiefs in the Arellano-Felix cartel in Tijuana.

Cardenas-Guillen, a former federal police officer in Mexico, faces 17 counts of drug importing and distribution, as well as three charges of threatening a federal agent and one count of money laundering. He faces a maximum sentence of life in prison if convicted.

Prosecutors said that the charges against Cardenas-Guillen originate in South Texas, but that he will be held in Houston, 350 miles from the border, out of concern for witness safety and border protection.

Cardenas-Guillen's lawyer, Robert Yzaguirre, said he hadn't had any time yet with his client.

The Guerreros, who face drug and money laundering charges, and Palma, who faces a charge of conspiracy to distribute cocaine, did not have attorneys with them.

Along with the suspected drug kingpins, Mexico also extradited 11 people wanted in the United States on charges including murder, drug trafficking, kidnapping and sex crimes. They also appeared in court Monday.

In all, 12 made their initial appearances in federal court, while three appeared in state district court in Houston. Most were set to have bond and arraignment hearings later this week. Cardenas-Guillen's was set for Feb. 1.

Once these hearings are done, the individuals are to be transferred to states where charges against them were originally filed, including California, New York and Colorado. Cardenas-Guillen is to remain in Texas, as will two other people. The Guerreros and Palma are to be transferred to California.

Mexico has recently shown more willingness to extradite drug lords, even those facing life in prison. A record 63 were sent to the U.S. in 2006 alone. However, it refuses to extradite anyone who would face the death penalty, which is illegal in Mexico.

Mexican President Felipe Calderon pledged Monday to wage a permanent war on organized crime.

"This is a permanent fight in which, unfortunately, many have lost their lives," he said. "We are fighting without pause so that these sacrifices will not have been in vain."