The Mexican navy says it has captured the main suspect in the murder of 145 people whose bodies were found in mass graves in the northern state of Tamaulipas earlier this month.
Omar Martin Estrada - known as El Kilo - is accused of being the local leader of the Zetas drug cartel in San Fernando, where the bodies were found.
A $1.2m (£735,000) reward had been offered for his arrest.
Most of the victims are thought to have been abducted from buses.
Mr Estrada is also accused of being involved in the murder of 72 Central and South American migrants whose bodies were found in the same area last year.
Five other suspects were also arrested.
Security forces had already arrested at least 16 suspected members of the Zetas in connection with the mass graves.
They have also been questioning 16 local police officers accused of protecting the criminals.
Most of the victims are thought to have been abducted from long-distance buses travelling north to the US border.
The motive for the murders is unclear, but there is speculation the cartel gunmen may have killed men who refused to join their ranks.
San Fernando near the US border is one of the deadliest spots in Mexico's drugs war, says the BBC's Julian Miglierini in Mexico City.
It is thought that the Zetas and their former allies, the Gulf cartel, have been fighting for control of the area because it is a route for smuggling drugs into the US, our correspondent adds.
Forensic scientists have been working to identify the bodies, some of which have been taken to Mexico City.
Hundreds of people whose relatives have gone missing have gone to see if they can identify their family members among the dead.
The Mexican government says around 35,000 people have been killed in drug-related violence since President Felipe Calderon began deploying troops to fight the cartels in December 2006.
More than 5,000 people have been reported missing, according to Mexico's human rights commission.