Mexican anti-drugs police believe they have arrested the boss of the country's largest cocaine smuggling cartel.
Mr Carrillo Fuentes is on a US list of wanted drug smugglers
Vicente Carrillo Fuentes, leader of the Juarez organisation, is thought to have made alliances with the notorious drugs baron Joaquin Guzman, or El Chapo.
A man using a different name but partially matching his description was arrested on Saturday in Mexico City.
Fingerprints and DNA samples are being sent to FBI laboratories in the US in an effort to confirm his identity.
The Juarez cartel is the only large Mexican drug gang that has not had one of its leaders arrested during major recent operations.
Mr Carrillo Fuentes is the brother of Amado Carrillo Fuentes, a Mexican drug lord who died in 1997 after botched plastic surgery to change his identity.
Police say the man arrested on Saturday was going under the name of Joaquin Romero Aparicio.
But authorities suspect Mr Carrillo Fuentes has also gone under the knife to avoid capture.
The authorities have been struggling to contain the drugs trade in recent months.
Last month more than 500 people were arrested in major operations against suspected cocaine and marijuana smugglers.
Army units have been stationed in the border town of Nuevo Laredo, where more than 70 people have been killed this year in drug-related violence, and local police have been accused of corruption and complicity with smugglers.
For its part, Mexico has called on the US to increase border security and stop weapons falling into the hands of drug gangs.