One of Mexico's most senior police officers, the acting Federal police commissioner, Victor Gerardo Garay, has resigned.
He stood down after an aide of his was accused of working for a leading drug cartel.
Mr Garay said he was stepping down to cooperate with the authorities.
Mexico is suffering from widespread drugs-related violence, with several thousand people shot dead this year as gangs battle for supremacy.
The news of the resignation of the highest ranking member of the Mexican federal police comes amid allegations that the drugs cartels may have infiltrated the highest echelons of the Mexican security and police forces.
The BBC's Emilio San Pedro says it appears to serve as further proof that there may be more truth in those allegations than the government has so far been willing to acknowledge.
"I am resigning because in the bloody fight against organized crime, it is our duty to strengthen institutions, which means it is essential to eliminate any shadows of doubt regarding me," the commissioner said.
His resignation comes as one of his top lieutenants is under investigation for having allegedly offered protection to members of the powerful Sinaloa cartel.
The Mexican press has alleged that the commission's office allowed the cartel to smuggle drugs through Mexico City's international airport.
It follows the arrest just last week of five members of the organised crime unit of the federal attorney general's office - who face accusations of having passed on information to members of another cartel.
This is more bad news for President Felipe Calderon, who has declared an all-out war against the drugs cartels and has been fighting to change the widely held perception in Mexico that the authorities are corrupt and not to be trusted.