By JAMES C. McKINLEY Jr.
 
MEXICO CITY, Feb. 6 - A major drug cartel had a spy inside the office of President Vicente Fox who fed one of its traffickers precise information about the president's movements for more than three years, prosecutors here say.

The attorney general, Rafael Macedo de la Concha, would not say which cartel was believed to be buying information from the insider, one of the president's aides, but federal agents investigating the leaks raided several houses on Saturday. Reforma, one of Mexico's most respected daily newspapers, reported that the houses belonged to Héctor Beltrán Leyva, a top lieutenant of Joaquín Guzmán, the most ruthless and wanted trafficker in Mexico.

The aide, Nahúm Acosta Lugo, was arrested in secret on Thursday after federal investigators looking into drug trafficking discovered evidence that he had been giving information about the president's private schedule to a particular drug trafficker. Mr. Acosta has been held since then at an undisclosed location in Mexico City while agents interrogate him. In court papers filed Saturday, he denied any wrongdoing.

Mr. Acosta, 42, is a political operative from a border town in Sonora, a major corridor for drug traffickers. Though his only professional experience had been as a grade-school teacher, he was hired in 2001 to be one of the advance men for trips by the president in what appeared at the time to be a run-of-the-mill patronage appointment.

Mr. Macedo de la Concha, the attorney general, said the actions of which Mr. Acosta was accused "could have put in risk the physical well-being of the president," though he hastened to add that investigators so far had not discovered evidence of an assassination plot.

"There are no facts or elements that would at this time make us worried that the security of the president of the republic is in risk," he said.

Still, security around President Fox was extremely tight on Sunday as he attended Mass at a church near his ranch in Guanajuato State. More than double the usual number of bodyguards kept reporters and local residents at bay, in a break with past practice, Radio Formato 21, a network of news stations, reported.

Investigators say Mr. Guzmán, known as El Chapo or Shorty, is largely responsible for an underworld war between cartels that has claimed dozens of lives in recent months. Mr. Guzmán, who bribed his way out of a high-security prison in 2001, has tried to move into territory controlled by imprisoned gangsters like Osiel Cárdenas, the head of the Gulf Cartel, and Benjamín Arellano, the boss of the Tijuana Cartel. One result has been bloodshed.

The discovery that drug dealers had bribed someone inside Los Pinos, as the Mexican presidential mansion is called, added to the sense of crisis that has gripped the government in recent months, as a tide of drug-related assassinations has swept the nation.

In the past three weeks, Mr. Fox has been forced to send troops and federal agents into the three major high-security prisons to search for weapons and re-establish order, after wealthy drug dealers had bribed wardens and guards to let them continue to run their networks from jail.

A spokesman for the president, Carlos Garza, said Mr. Acosta was privy to details about the president's trips and sometimes traveled with him, though he was usually sent ahead to check sites and handle logistics. Mr. Garza maintained that Mr. Acosta was not always informed of the precise route of the president's motorcade, nor of all the security measures that would be taken.

Mr. Fox, who is scheduled to go to Spain for a state visit on Monday, has said nothing about the arrest. Mr. Garza said the president still had confidence in the other officials in his office who organize his public events and trips. The president canceled a trip to Ciudad Juárez and Tijuana that was to take place last week, but Mr. Garza said the decision had nothing to do with Mr. Acosta's arrest.

Mr. Acosta got his start in politics in his hometown, Agua Prieta, where he ran unsuccessfully for mayor in 2000. He also developed close relationships with several politicians in the National Action Party who helped Mr. Fox win election that year. He himself had switched to Mr. Fox's party in 1999.

When Mr. Fox took office, he appointed several of his party members from Sonora to important positions: Alfonso Durazo became his personal secretary and Manuel Espino his trip coordinator. Both later left the government.

Mr. Garza said it was Mr. Espino who brought in Mr. Acosta as his top aide, and Mr. Acosta remained in the position after Mr. Espino left the government to seek the chairmanship of the national party.