(CNN) -- Mexican authorities have arrested seven more people in connection with the killings of 72 migrants in northeastern Mexico, said Alejandro Poire, the president's spokesman for security issues.
The seven suspects are alleged members of the Zetas drug cartel, which police believe was behind last month's massacre, Poire said.
Four of the suspects were detained last week after a clash with Mexican marines. In that operation, police recovered weapons, found two bodies and freed three kidnapping victims. Investigators were determining whether the two bodies belonged to migrants as well, Poire said.
The confrontation happened near San Fernando, the town closest to the ranch where the bodies of the migrants were found.
In a separate incident, authorities captured three others suspected of taking part in the killings, he said.
The suspected Zetas were in the custody of the federal attorney general's office.
The announcements of the arrests come after the identification of seven additional suspects in the same case, all but one of whom were dead. Three had died in a clash with the military and three others were found dead. The seventh suspect is in custody.
Poire said there has been no evidence to weaken the government's initial theory that the migrants were killed when they declined to join the cartel to fight their rivals.