Hours earlier, investigators in the Pacific coast state of Sinaloa found at least a dozen skeletal remains in a series of pits, while officials in neighboring Sonora pulled four bodies from two pits, authorities said Wednesday.
The graves in Sinaloa were discovered by a farmer who reported unusual odors coming from a field in the township of Ahome, near the state line with Sonora, state prosecutors' spokesman Martin Gastelum said.
He said that seven pits were found late Tuesday and excavations continued Wednesday. Two sets of remains appeared to belong to women who had been reported missing in October. The state is the home base of Mexico's most powerful drug gang, the Sinaloa cartel.
The Sonora pits were discovered Wednesday in Nogales, a city across the U.S. border from Nogales, Arizona, Sonora state police said in a statement. One pit contained the body of a man and the second held the corpses of three men.
All the victims appeared to be about 30 to 35 years old, and the bodies appeared to have been buried between 10 and 15 days ago, the statement said. Mexican drug cartels have been blamed for a number of such mass graves, in which gangs deposit the bodies of kidnap victims or executed rivals.
In Tamaulipas, federal authorities say 17 suspects tied to the brutal Zetas gang have been detained in relation to the killings, some of whom have purportedly confessed to abducting passengers from buses and killing them.
The graves have been found in the township of San Fernando, the same area of Tamaulipas where investigators found the bodies of 72 mostly Central American migrants massacred by suspected drug cartel gunmen last August.
Police say witnesses in the latest killings have told them that gunmen pulled the victims, mostly young men, off passenger buses traveling through the San Fernando area in late March. The motive for the abductions remains unclear, though prosecutors suggest the gang may have been forcefully seeking recruits.
San Fernando is a town about 90 miles (145 kilometers) south of Brownsville, Texas, on a well-traveled stretch of highway. The Zetas and rival Gulf Cartel are fighting in Tamaulipas over lucrative drug transit routes to the U.S.
In the northern industrial hub of Monterrey, a spokesman for state police said Wednesday that five suspected drug cartel members and a female motorist were killed in a shootout between soldiers and the suspects late Tuesday.
The spokesman, who was not authorized to be quoted by name, said the five suspects -- including one woman -- were traveling in a sport utility vehicle on an expressway and ignored an order to stop. In their bid to escape, they opened fire on soldiers and apparently tossed a hand grenade that hit a van, which exploded but whose driver was able to escape without major injuries.
However, a woman traveling in another SUV was hit by crossfire in the running gunbattle and died. Her 8-year-old daughter suffered a gunshot wound to the leg and is in stable condition.