Mexico's federal police rescued 51 people, including six Chinese and 18 Central American migrants, who were captive in the same border city where more than five dozen people were freed last week, authorities said Monday night.
Police investigators were led to the latest group while following up on last week's rescue of 68 people in Reynosa, across from McAllen, Texas, federal officials said.
Authorities gave no motives or identified the kidnappers or drug gang involved, but drug cartels have been accused of pulling people off buses or abducting them from bus stations.
Police said among those freed were six Chinese, 14 Guatemalans, two Hondurans, two Salvadorans and 27 Mexicans. They were captive inside a house in a neighborhood of Reynosa.
The group rescued last week included 12 Central American migrants who were presumably kidnapped by the Gulf carte.
Federal police said Monday that they detained four municipal officers in the kidnapping of the 68 people. The four officers were taken to Mexico City and will be presented before a federal judge. Two other men allegedly involved in the kidnappings were detained last week.
Mass kidnappings of migrants have become common in the state of Tamaulipas, where formerly allied Zetas and Gulf drug cartels are battling over turf. Authorities believe the Zetas may be forcefully recruiting migrants.
Tamaulipas authorities have been unearthing dozens bodies from pits in San Fernando, 80 miles (130 kilometers) from the U.S. border. So far, the total of bodies exhumed this month stands at 177.
In the same municipality, police in Tamaulipas found the bodies of 72 Central American migrants shot to death last August.
Authorities in Durango, another northern Mexican state, have also been unearthing remains, to the point that officials have had to rent space to hold bodies to await identification.
Investigators said that they exhumed skeletal remains of 17 people at a vacant lot where a total of 54 bodies have been recovered in the state capital, also called Durango.
State prosecutors said the excavation would continue Tuesday.
None of the bodies have been identified, and authorities have offered no motives for the killings.
A fight between the Zetas and Sinaloa cartels is blamed for drug violence in Durango state.
Before investigators found the mass grave, they recovered 10 complete bodies, three headless bodies and four severed heads at a ranch, also in the capital.