The longest-serving Black U.S. congressman has died at 90 years old.

According to The Washington Post, a family spokesperson confirmed John Conyers Jr.'s passing at his home in Detroit on Sunday. He died of natural causes. 

Conyers co-founded the Congressional Black Caucus and helped create a national holiday in honor of Rev. Martin Luther King Jr., the Washington Post reported.

The Michigan representative, who was first elected to Congress in 1964, became known for supporting progressive causes.

The NAACP released a statement Sunday, saying the nation lost a fervent champion for equal justice. 

"Conyers served for five decades in Congress and was widely known as the Dean of the House," the NAACP said. "Elected in 1964, Rep. Conyers was an unmovable force when it came to advancing equality and civil rights." 

Conyers chaired the House Oversight Committee from 1969 to 1995. In 2007, he became the first Black person to serve as chair of the House Judiciary Committee. The NAACP honored Conyers in the same year, giving him the Spingarn Medal for his dedication in the fight for justice.

"His life-long career of pushing this nation toward justice, fairness, and liberty left an indelible impression on the world," the NAACP said.

Conyers' career was also marked by allegations of sexual harassment. According to BuzzFeed's report in 2017,  Conyers' female staff members said he asked them to have sex and touched them inappropriately. Following the allegations, Conyers denied the accusations. He resigned in 2017 after 52 years in office.

The Detroit native voted against the USA Patriot Act and pushed for George W. Bush to be impeached, saying he misled the country during the 2003 U.S. invasion of Iraq, the Post reported. Conyers also criticized Barack Obama in 2011.  

"We keep getting a longer and longer list of things he wanted to do, wished he could do more about, and is of course having a big problem," Conyers told reporters. "The only thing that saves him, of course, is that there doesn't seem to be anybody to run against him next year."

According to the New York Times, Conyers participated in impeachment inquiries against both Presidents Richard M. Nixon and Bill Clinton — the only House Judiciary Committee member to do so.

He is survived by his wife, Monica, and sons, John and Carl.