The mass escape on Friday was the second in seven months from the prison, known by its Mexican acronym, CEDES, the San Antonio Express-News reported. In December, the warden disappeared when 151 inmates broke out.
Mexican authorities issued a brief press released that said 59 inmates had gotten out. Some Mexican news agencies said the actual number was 66, the newspaper said.
"The situation is currently under control and the facts are being investigated in a coordinated manner with local and federal authorities," the government statement said.
Webb County Sheriff Martin Cuellar in Laredo, Texas, across the border from Nuevo Laredo, said the escapees were members of the Zetas and the inmates killed Friday were in the rival Gulf Cartel. Cuellar said the December break was also organized by the Zetas and included several inmates who had been charged with killing members of other gangs.