Press freedom groups are condemning an attack on a television station in northern Mexico and demanding a quick and thorough investigation of the incident.

On Tuesday, masked gunmen in two pickup trucks threw a grenade and opened fire on the studio of the Televisa network in Monterrey. No one was injured, but the attackers left behind a note warning the station about its coverage of drug gangs.

The Inter American Press Association issued a statement Wednesday, saying it is clear that organized crime keeps sending such "messages" to generate fear and force the news media and individual journalists to resort to self-censorship.

Paris-based Reporters Without Borders said the attack shows organized crime is targeting Mexico's national and local media.

The Committee to Protect Journalists calls Mexico one of the world's deadliest nations for journalists, with 21 killed since 2000. It says drug traffickers are thought to be behind most of the slayings. Drug-related violence in Mexico claimed more than 5,000 lives in 2008.

In a report last year, the U.S.-based CPJ said there is mounting evidence that local officials have punished journalists who report on the alleged links between government officials and drug gangs.

The report said seven journalists have disappeared in Mexico since 2005 and many of them were in the process of investigating relationships between the government and organized crime. It said relatives and colleagues of several missing journalists said they believed local public officials played a role in the disappearances.

Some information for this report was provided by AP.