The Mexican navy says the leader of the powerful Zetas drug cartel, Heriberto Lazcano, was apparently killed in a shootout in a northern border state.
A navy statement says two alleged gang members were killed in a gunfight with naval forces in the northern state of Coahuila on Sunday, and there was "strong evidence" that one of them was Lazcano.
However, it says more forensics tests need to be carried out to confirm the identification.
The Zetas are considered one of Mexico's main drug cartels and are known for decapitating and dismembering their enemies.
The United States has offered a $5 million reward for information leading to Lazcano's arrest.
Tens of thousands of people have been killed in Mexico since President Felipe Calderon began a military crackdown on drug gangs in late 2006.
On Monday, naval authorities announced the arrest of a Zetas commander allegedly linked to the massacre of 72 immigrants, and the murder of an American man and a Mexican police chief. The commander is also linked to two massive jail breaks.
Navy spokesman Jose Luis Vergara said Salvador Alfonso Martinez Escobedo is allegedly responsible for the mass graves found in the state of Tamaulipas with more than 200 bodies.