Mexico's National Human Rights Commission says 56 journalists have been killed in the past nine years and most of the murders remain unsolved.

It says eight reporters went missing and seven newspaper offices were attacked with explosives over the same period.

The commission said in a news release Tuesday that authorities have been negligent in investigating and prosecuting the attacks.

It didn't say who is believed to be behind the attacks on journalists, but news media have been frequent targets of Mexico's drug cartels. Some of the slain journalists were investigating drug trafficking, crime or corruption.

Several international media watchdog groups have named Mexico the most dangerous country in the Americas for journalists.