MEXICO CITY—A day after 44 inmates died in Mexico's worst prison riot, authorities said they believed the massacre was a cover for the escape of 30 drug-gang members.

Nuevo León Gov. Rodrigo Medina said that 30 inmates, all members of the Zetas drug cartel, used the massacre on Sunday as cover for an escape from Apodaca state prison, a few miles from the state capital of Monterrey.

"Without a doubt there was premeditation," said Mr. Medina, speaking at a news conference. "This was planned."

Mr. Medina said all the dead prisoners were members of the rival Gulf Cartel. The Zetas and the Gulf Cartel have been warring for two years for control of drug routes and lucrative drug markets, especially in the northeastern Mexican states of Nuevo León and Tamaulipas.

Mr. Medina said prison personnel appeared to have been involved in the massacre and escapes. He said four top prison officials had been fired from their jobs, as had 18 guards on duty at the time of the incident. All were being investigated for complicity, he said. As police and soldiers combed the state searching for the escaped prisoners, Mr. Medina said the state was offering a reward of about $800,000 for information leading to their capture.