Two federal police and a bystander were killed and 15 others injured in a clash with gunmen in Mexico's western state of Michoacan, authorities said.
The two police officers died Sunday night when gunmen opened fire on their patrol in the town of Apatzingan, the Michoacan Attorney General's Office said. A woman bystander also died at the scene, the report said.
The report said police tried to stop several vehicles of heavily armed men, who opened fire in response. Seventeen agents were injured, and two later died from their injuries.
Michoacan is considered a stronghold of a cartel known as La Familia, one of Mexico's most violent drug-trafficking groups.
Police in Tijuana, meanwhile, found three bodies in a clandestine grave after being led to the site by a man who allegedly acted as a hit man for a key drug trafficker.
The bodies - belonging to a woman and two men - were discovered following the arrest of Higinio Melero Lopez, said Fermin Gomez, the deputy attorney general of Baja California state, where the border city is located.
Lopez led police to the site after confessing that he had acted as an assassin for Raydel Lopez Uriarte, an alleged leader of a cartel headed by Teodoro Garcia Simental.
Lopez Uriarte and Garcia Simental were both arrested early this year, and Melero told police the three bodies belonged to drug dealers who refused to work for Garcia Simental's gang.
It was the second clandestine grave found in the 3 de Octubre neighbourhood of Tijuana, once reputedly a stronghold of Lopez Uriarte. Police found two bodies in a grave in August after arrested four other alleged hit men for Lopez Uriarte.
Authorities said the bodies were all in an advanced state of decomposition and it was unclear how long ago they were killed.