Police in the suburbs of Mexico City found a total of five decapitated bodies on Tuesday accompanied by written messages of the kind frequently left by drug gangs.
The bodies were found at two sites in Mexico state, which surrounds the capital. While Mexico City has been spared most of the country's drug violence, executions have occurred in communities just outside it.
Mexico state chief prosecutor Alfredo Castillo says four of the bodies were found in a compact car. Their heads were also found in or around the vehicle.
Another decapitated body was found Tuesday in another suburb in two plastic bags. At both sites police found messages signed "HCC," an apparent reference to a drug gang.
Also Tuesday, prosecutors in the northern state of Durango announced that six more sets of skeletal remains had been found in continuing excavations at mass graves, bringing the total number of bodies found in a month-long search of the sites to about 110.
The sites around the state's capital, also called Durango, are pits where drug gangs are believed to have buried their victims.
The graves in Durango are the second such discovery in a month. A total of 183 bodies have been unearthed in 40 pits in the northeastern border state of Tamaulipas.
Mass graves have become an increasingly common discovery in Mexico's brutal drug war, which has claimed more than 34,600 lives since President Felipe Calderon deployed thousands of federal security forces four years ago to fight traffickers. The offensive led to a splintering of the country's cartels and increased gang fighting over territory.
State authorities across Mexico have sent reports of missing persons to Tamaulipas and families have lined up at morgues to give DNA samples. The process has been slow, with only two bodies identified so far.
Authorities in Durango state say the discovery of mass graves there has not brought out many relatives of missing people, perhaps because families are too frightened to come forward.
Alejandro Poire, the federal government spokesman for security issues, defended the government's offensive against drug cartels, saying there was no evidence it has unleashed increased violence.
"The intervention of federal authorities, far from increasing the tendency to violence, has prevented it from increasing," Poire told reporters Tuesday.
"It is not true that the capture or death of criminal leaders has caused violence to increase," he said.
In an interview with the Televisa television network, Calderon said he did not support declaring a state of emergency in the states worst hit by drug violence, like Tamaulipas, but said there might be other legal remedies for fighting cartels in areas where they have gotten out of control.
"What we need to do is not suspend peoples' rights," Calderon said.
"Perhaps there are other constitutional mechanisms," he said, mentioning a clause that would allow for the replacement of local officials if they are proved to have lost control of the situation.
Federal prosecutors have already arrested 16 local police officers in San Fernando, Tamaulipas, where the massacre occurred, for allegedly protecting members of the Zetas and covering up the kidnappings of bus passengers and others who traveled on a highway connecting San Fernando to the U.S. border.
While Calderon said "the country's future is in danger," he acknowledged that he did not think he could get the kind of bipartisan support needed from Mexican political parties for such a move.