Prosecutor for crimes against freedom of speech Laura Angelina Borbolla says she is aware of only one case involving the slaying of a journalist that reached sentencing.
Borbolla testified at a congressional hearing Tuesday that her unit has identified 74 suspects in reporters' killings, but she didn't say if any of them had been detained.
Press advocates have long called Mexico one of the most dangerous nations for reporters.
The Committee to Protect Journalists previously has said that 48 were killed or disappeared from December 2006 to the end of 2011, and five more were murdered this year. Mexico's human rights commission lists 81 journalists killed since 2000.