"It's irresponsible that without doing a minimal investigation, authorities immediately linked the killings a vengeance by members of organized crime," it said.
Four journalists have been killed in Coahuila and two more have gone missing since 1989. None of the cases have been solved, the association said.
Press advocates have long called Mexico one of the most dangerous nations for reporters. But there isn't a single, agreed-upon figure on crimes against journalists.
The Committee to Protect Journalists says in its latest report published in February that 12 Mexican journalists went missing in 2006-2012 and that in the same period 14 were killed because of their work. Mexico's human rights commission lists 81 journalists who it says have been killed since 2000.
In 2012, Mexico's special prosecutor for crimes against freedom of expression said 67 journalists had been killed and 14 had disappeared in the country since 2006.
On Thursday, the lower house of congress approved a bill that would allow journalists to request that federal prosecutors and federal judges investigate attacks on them, and to establish cases in which such federal intervention would be obligatory. The bill was previously approved by the senate and has now been sent to the president for his signature.