Mexican police say a suspected drug cartel leader they arrested last week has confessed to ordering the killing of 1,500 people in northern Chihuahua state.
Jose Antonio Acosta Hernandez is also a suspect in the murder of a United States consulate employee last year near a border crossing in Ciudad Juarez.
Felipe Calderon, the Mexican president, said on Sunday that the capture was "the biggest blow" to organised crime in Ciudad Juarez since he first sent about 5,000 federal police personnel to the city in April 2010 in a bid to curb violence in one of the world's most dangerous cities.
Acosta, 33, was caught on Friday in the northern city of Chihuahua, said Ramon Pequeno, head of the federal police's anti-drug unit.
The arrest was not confirmed until Sunday, just before Acosta was displayed to the media in Mexico City.
He was limping as he was brought before the cameras, escorted by two masked federal police officers.
Acosta, who is nicknamed "El Diego", told federal police that he had ordered 1,500 killings, Pequeno announced at the news conference.
Investigators say that he was also the mastermind behind an attack that killed a US consulate employee, her husband and the husband of another consulate worker, in Ciudad Juarez.
US prosecutors say they want to try him in that case, and a federal indictment filed in the western district of Texas names Acosta and nine others as conspiring to kill the three US citizens.
Pequeno said that he expects an extradition request to be filed by the US government.
Mexican authorities have identified Acosta as the head of La Linea, a gang of hit men and corrupt police officers who have been acting as the enforcers of the Juarez cartel.
Pequeno said that Acosta acknowledged that he had ordered such crimes as the detonation of a July 2010 car bomb and a massacre that killed 15 people at a birthday party. Both events took place in Ciudad Juarez.
The Juarez cartel, allegedly led by Vicente Carrillo Fuentes, has been losing ground in recent times to the Sinaloa drug trafficking organisation, headed by Joaqiun "El Chapo" Guzman.
The two groups have been locked in a three-year battle over the border city's smuggling corridors.
Fuentes and Juan Pablo Ledezma, allegedly his top lieutenant, remain at large.
Acosta is a former state police officer, and built a criminal empire out of leading a gang of contract killers for the Juarez cartel and extorting businesses, as well as carrying out kidnappings for ransom, said Tony Payan, an expert on the drug war at the University of Texas at El Paso.
"This is an enforcer and the financial arm of the Juarez Cartel,'' said Payan, whose research comes from both newspaper accounts and people living in Ciudad Juarez.
Payan said Acosta was able to gather intelligence using informants from within local police forces, given his own past experience with law enforcement.
He said that Acosta's arrest could reduce the number of murders in Juarez, where more than 3,000 murders were recorded last year.
"He was a very hands-on manager that was practically involved in the management and organisation, personally brokering every single activity and every single murder," Payan said.