Prominent Black Lives Matter activist DeRay Mckesson arrested at Alton Sterling protest in Baton Rouge _lowres

Police arrest activist DeRay McKesson during a protest along Airline Highway, a major road that passes in front of the Baton Rouge Police Department headquarters Saturday, July 9, 2016, in Baton Rouge, La. Protesters angry over the fatal shooting of Alton Sterling by two white Baton Rouge police officers rallied Saturday at the convenience store where he was shot, in front of the city's police department and at the state Capitol for another day of demonstrations. (AP Photo/Max Becherer)

While a federal appeals court has allowed a Baton Rouge police officer's lawsuit against a Black Lives Matter protester to go forward, one judge said it was unlikely the suit would ultimately succeed.

His argument: First responders can't sue when injured in the line of duty because they assumed the risk when they took the job.

The lawsuit has attracted attention nationwide because it concerns protests held after the fatal police shooting of Alton Sterling in 2016. 

The officer, identified in court records only as John Doe, doesn't claim that Black Lives Matter organizer DeRay Mckesson threw a chunk of concrete that seriously injured him. However, he argues Mckesson was a prime leader and organizer of the July 9, 2016 protest and should therefore be held liable. Mckesson argues he is protected by his First Amendment right to protest. 

Multiple courts have tackled questions about the validity that defense. But Fifth U.S. Circuit Court of Appeals Judge James Ho wrote in a concurring opinion filed Tuesday that Mckesson would likely win the lawsuit for a different reason.

"Police officers and firefighters dedicate their lives to protecting others, often putting themselves in harm's way," he wrote. "What's more, police officers and firefighters assume the risk that they may be injured in the line of duty. So they are not allowed to recover damages from those responsible for their injuries."

Ho cited a common law rule called the "professional rescuer doctrine." He predicted that the suit will fail at the district court level if the defendant's attorneys raise that argument.

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Ho's opinion came as the Fifth Circuit rejected Mckesson's attempt to overturn previous rulings saying the officer can sue him. It's the latest development in a winding legal path.

U.S. District Judge Brian Jackson, of Baton Rouge, previously dismissed the lawsuit in its entirety, citing Mckesson's First Amendment rights and noting the Black Lives Matter group was too loosely organized to sue.

But the 5th Circuit reinstated the suit last year and said the officer can sue Mckesson on the grounds that he acted negligently by leading people to block a highway outside BRPD headquarters. The appellate court then issued a second ruling in the case in August, saying the officer's complaint should not have been dismissed on First Amendment grounds, even if he was injured during a political protest.

In its third written opinion issued last month, the 5th Circuit explained that the injured officer has plausibly alleged that Mckesson breached his duty of reasonable care in the course of organizing and leading the Baton Rouge demonstration. 

The U.S. Supreme Court can now choose whether to hear the issue. If it does not, the case would go back to the start and the trial would proceed.

If it does, Ho argues the professional rescuer doctrine means Mckesson would win.

"This doctrine would seem to require immediate dismissal of this suit. After all, there is no dispute that the officer was seriously injured in the line of duty — specifically, while policing a Black Lives Matter protest that unlawfully obstructed a public highway and then turned violent," he wrote. "The officer deserves our profound thanks, sympathy, and respect. But his case would appear to fall squarely within the scope of the doctrine."

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