MEXICO CITY -- In a spiraling probe into corruption at top levels of Mexican law enforcement, authorities yesterday said the nation's former anti-drug chief had accepted $450,000 to tip off traffickers.

Noe Ramirez Mandujano, a veteran federal prosecutor who headed an elite organized-crime unit known as SIEDO, was arrested on suspicion of passing intelligence to drug gangsters based in the northwestern state of Sinaloa, Mexican Attorney General Eduardo Medina Mora said.

Mr. Ramirez, 47, who served for 20 months before quitting in July, is the highest-ranking official arrested as part of the government's investigation into infiltration of police agencies by drug traffickers. The charges against him are the most serious against a Mexican anti-narcotics official since the country's former drug czar was arrested on charges of helping a Ciudad Juarez-based cartel in 1997.

Six other officials and agents from SIEDO, a division of the attorney general's office that investigates drug smuggling, arms-trafficking and other mafia crimes, already face charges of leaking intelligence to the Sinaloa group.

Mr. Medina Mora said a protected informant told authorities that he had paid Mr. Ramirez a total of $450,000 as part of a monthly payoff scheme, "in exchange for providing information about investigations and ongoing actions" against the Sinaloa-based smugglers.

The attorney general said Mr. Ramirez voluntarily appeared before prosecutors to answer the accusations, but afterward there was still sufficient cause to detain him.

Mr. Medina Mora did not specify what information Mr. Ramirez allegedly passed to the gang. He said information was given to two Sinaloa factions: the Beltran Leyva brothers and Zambada brothers.

The charges, if true, represent a major setback for President Felipe Calderon's war on Mexican drug cartels, which has been a centerpiece of his 2-year-old administration. Mexico is awash in drug violence, with more than 4,000 dead this year, according to unofficial counts by the nation's media.

The allegations put a new dent in the reputation of SIEDO, which in the past had been considered trustworthy by U.S. officials. Further evidence of high-level cartel infiltration could leave U.S. agents more wary about sharing information with the agency.

Mr. Ramirez's arrest will likely further undermine public confidence as well. Mexicans are increasingly weary of the killing and long ago became accustomed to corruption charges against top police officials.

The offensive, which has sent 45,000 federal troops and 5,000 federal police into the streets, has yielded arrests of several high-profile trafficking figures and big seizures of drugs, cash and weapons. But the offensive has yet to crush any of the major drug mafias.

Moreover, Calderon administration officials in recent months have confronted an embarrassing string of arrests of ranking police officers and prosecutors assigned to the drug fight. Earlier this week, authorities announced that they had detained Mexico's liaison to Interpol as part of the probe into leaks to the cartels. He was the third top-level federal police commander ordered held in recent weeks.