Mexico said it is investigating what caused two infants to die after receiving government vaccinations days ago in the predominantly poor southeastern state of Chiapas.

The incident began on Friday when 52 young children were inoculated with three vaccines in Simojovel, Chiapas, as part of a free national health program. Within 72 hours, more than 30 of the infants and toddlers became sick.

Although the batch is believed to have been isolated to Simojovel, vaccination programs were halted nationwide on Saturday.

Five of the children who became sick after the vaccinations were released from a hospital in the state capital Tuxtla Gutierrez but 24 remained hospitalized, officials said Tuesday morning.

Six of the children were still in grave condition, said Mexico's social service institute, IMSS.

The children received doses of three separate vaccines, for hepatitis B, tuberculosis, and rotavirus. Mexican health authorities said they believe the hepatitis vaccine is the culprit, but have not yet confirmed the theory.

According to witnesses, the young children fell into a 12-hour stupor after the vaccinations. Two infants did not wake up and were pronounced dead.

"We are working at a frantic pace to figure out what happened," Jose Antonio Gonzalez, the head of IMSS, said in a radio interview. He said the batch in question has been located and isolated.

Mexico's human rights commision said it was launching an investigation into the deaths. Gonzalez promised the agency would behave with "transparency" as it figures out what happened.

He said IMSS applies 35 million vaccines throughout the country every year without incident. Vaccinations resumed this week, the IMSS said in a statement Tuesday.

"The first thing is to determine if there was a problem with the handling of the vaccines," Gonzalez said. "So far that appears to be the case."