A hashtag can be a powerful force in corralling social movements of the digital age, #blacklivesmatter being a case in point. But a neither a hashtag nor a social movement can be sued, a federal judge in Louisiana ruled Thursday after a Baton Rouge police officer — whose name has not been made public — filed a lawsuit against Black Lives Matter and prominent activist DeRay Mckesson.

In the suit, the officer said he was seriously injured during a protest last year that "turned into a riot," when a piece of concrete or "rock-like substance" thrown by an unidentified demonstrator hit him.

The plaintiff argued Black Lives Matter could be sued, U.S. District Judge Brian A. Jackson writes, because it is a "national unincorporated association" with Mckesson "its leader and founder," who was "in charge of the protests."

But in throwing out the suit, Jackson writes,"'Black Lives Matter,' as a social movement, cannot be sued, however, in a similar way that a person cannot plausibly sue other social movements such as the Civil Rights movement, the LGBT rights movement, or the Tea Party movement."

Activists launched #blacklivesmatter online some four years ago, following the shooting death of Trayvon Martin. But the hashtag quickly spilled into the streets, morphing into a movement against police brutality, racism and injustice.

"It's organized. They have meetings. They solicit money. They have national chapters," the police officer's attorney Donna Grodner said of Black Lives Matter in June, reports The Associated Press. "This shows a level of national organization."

But in his decision, Jackson notes that the plaintiff, identified as John Doe, "is attempting to sue a hashtag ... For reasons that should be obvious, a hashtag — which is an expression that categorizes or classifies a person's thought — is not a 'juridical person' and therefore lacks the capacity to be sued."

While a movement or a thought cannot be sued, Jackson notes that an entity associated with a movement can be sued, such as the NAACP, the Human Rights Campaign or Tea Party Patriots.

As for Mckesson, Jackson writes that he was exercising his constitutional right to association and protected speech in the protest and there is no evidence that he incited violence.

"It's clear that I did nothing wrong that day and that the police were the only violent people in the streets," Mckesson said after learning of the ruling, reports AP.

The demonstration at the center of the lawsuit took place July 9, 2016, in response to the shooting death four days earlier of Alton Sterling, a 37-year-old black man by a white Baton Rouge police officer.

Police were responding to reports of a man selling CDs and threatening somebody with a gun. Cellphone video showed officers wrestle Sterling to the ground outside a convenience store. An officer shouts, "He's got a gun!" before shots ring out. A .38 caliber revolver was found in Sterling's pocket after the shooting, according to the Justice Department. But whether Sterling was reaching for the gun has been disputed.

After Sterling's death, protesters took to the streets across the country. In Baton Rouge, more than 100 people were arrested at the July 9 rally, including Mckesson, who told The New York Times that peacefully assembled protesters were unfairly targeted. Police charged most of them with obstructing the road.

The protest was also the backdrop of a photograph that went viral, showing a lone woman, dress afloat, calmly confronting approaching officers.

After a near yearlong investigation, the Justice Department decided not to prosecute the officers, saying that there was not enough evidence to prove they violated Sterling's civil rights.

Sterling's children have filed a civil lawsuit against Baton Rouge seeking unspecified damages in his death.

After Thursday's ruling, Mckesson told AP, "The movement began as a call to end violence and that call remains the same today.

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