The attorney general's office was investigating the incident, a spokeswoman for the city's penitentiary system said.
A fire at a prison in eastern Mexico City on Thursday morning killed at least three men and injured seven others, the city government said in a statement.
Officials were still working to determine the cause of the fire, which was detected in a sleeping area at the prison in the Iztapalapa neighborhood at 5:25 a.m. local time, the statement said.
The three inmates killed in the fire were identified as Luis Enrique Tejeda, Mauricio Espindola and Carlos Enrique Perez, officials said. The seven injured were taken to hospitals to be treated for smoke inhalation.