Joel Simon is the executive director of the Committee to Protect Journalists. The views expressed in this commentary are his own. This is the next installment in the CNN Opinion series on the challenges facing the media, which is under attack from critics, governments and changing technology.

(CNN) The murder of five employees of the Capital Gazette has elicited an outpouring of grief and horror that such an attack could take place here in the United States. Indeed, yesterday's shooting represents the single deadliest attack on the US media in recent history, according to the research carried out by the Committee to Protect Journalists.

These murders highlight another important reality, which has been true in the US and around the world: local journalists are especially vulnerable to violence and are often targeted from within the communities they cover. This may have to do with the intimacy of the relationship. The journalists and their subject often know each other and the coverage may be felt more personally because it is seen by friends and neighbors.

At the same time, violence against the US media is hardly unprecedented. Last month, a local music journalist, Zachary Stoner, was murdered in Chicago. While the motive behind that killing is not yet known, Stoner reported receiving threats over his reporting on the death of a local teenager. A decade ago, community journalist Chauncey Bailey was gunned down in Oakland, California, in reprisal for exposés on a local restaurant called Your Black Muslim Bakery and its ties to violent crime in the community.

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While the US likes to think it is largely immune to the cycle of violent reprisal that characterizes the work of the media in so many parts of the world, the murders at the Capital Gazette serve as a terrible reminder that the right to free expression is both precious and vulnerable, even in this country. Expressed another way, the US is now the third deadliest country for journalists in 2018, behind only Afghanistan, where 10 journalists have been killed because of their work, and Syria, where the number is five.

In 1994, CPJ published a report called " Silenced: The Unsolved Murders of Immigrant Journalists in the United States ." The journalists, who wrote in variety of languages including Spanish, Haitian Creole, and Vietnamese, were all murdered in reprisal for their reporting. In November 2015, ProPublica published an investigation into the murders of five Vietnamese-American journalists killed between 1981 and 1990. The report linked the killings to a Vietnamese political organization called The Front that was alleged to be operating a death squad in the United States.

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