Attacks against journalists increased by nearly 22 percent in Mexico last year, with eight media workers killed and nearly 400 assaulted, a press rights group said Thursday.

The report by Article 19 said that the organization has observed a "deterioration of freedom of speech" since President Enrique Pena Nieto took office in December 2012.

The number of attacks has risen over the years, from 207 in 2012 to 326 in 2014 and 397 last year, the report said.

Of the eight killed in 2015, seven were reporters who were murdered "possibly in connection with their journalistic work" while the eighth victim was an administrative employee of a news organization.

Some 41.5 percent, or 165, of the 397 attacks last year were perpetrated by federal, state and municipal officials, the document said. Another 35 of the assaults were blamed on organized crime groups.

At least 55 media workers have been killed since 2009, Article 19 said.

Mexico has become one of the world's most dangerous places for journalists. Reporters Without Borders says 91 have been killed and 17 have gone missing since 2000.