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Rap stars Killer Mike, 21 Savage, Meek Mill and others filed a Supreme Court brief asking justices to review the conviction of a Pennsylvania rapper for his lyrics.

The artists are joined by academics and music industry professionals in the amicus brief filed Wednesday in the case of Jamal Knox, 24, known as Mayhem Mal, who went to prison for threatening two police officers by name in a 2012 song, titled 'F — the Police.'

The Pennsylvania Supreme Court upheld the rapper's conviction in August, finding that the lyrics of the song are not protected under the First Amendment.

But the brief filed this week argues that there is a difference between song lyrics and an expressed intent to take action.

"If rappers killed everyone they said they did in their songs, there would be no one left alive," professor Erik Nielson, one of the brief's principal authors, told NBC News.

Meek Mill performs during the 68th NBA All-Star Game at Spectrum Center on Feb. 17, 2019 in Charlotte, North Carolina. Jeff Hahne / Getty Images file

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The language directed at the officers in the lyrics was a form of artistic expression and "political expression that falls well within the scope of activity protected by the First Amendment," the brief states.

Nielson, an associate professor of liberal arts at the University of Richmond, said he has been following Knox's case since his arrest in 2012, and has been researching the use of rap lyrics as evidence in criminal cases for more than half a decade.

He said that rap lyrics are often used as evidence of someone's involvement in an underlying crime. But Knox's case is different, he said.

"With Jamal Knox, it is a smaller subset of cases, where the words themselves are the crime," Nielson said. "If we were being consistent about this, country singers would also be behind bars."

"There is a certain absurdity to it, but at the time same time, it's frightening because there is a racial double standard," the professor claimed.

The Supreme Court brief alleges such a double standard on music lyrics.

Nielson said the "continued ignorance" of the judiciary overall on hip-hop and rap is "partly generational but at a certain point, it becomes willful ignorance if you go out of your way to incarcerate young, black men, without learning the basic conventions of the genre."

Alex Spiro, a prominent music industry lawyer who filed the brief, told NBC News that the way judges determine these cases is "troubling and problematic."

He said the bias against rap and hip-hop "can lead to two people, a white country singer and a black, inner-city rapper, saying the exact same language, and one can be prosecuted, jailed and convicted, and the other one can have no moment from it."

Spiro said the brief is meant to serve as "a primer on rap music and hip-hop" for Supreme Court justices, who range in age from 51 to 85.

"What we would want to do is change the law," he said.