Actor Michael K. Williams, who was best known for playing Omar Little on "The Wire," died Monday.

Williams was found dead Monday afternoon by family members in his Brooklyn apartment, New York City police said. He was 54.

His death is being investigated as a possible drug overdose, the NYPD said. The medical examiner is investigating the cause of death.

Hollywood was shocked by the Emmy-nominated star's sudden death and penned moving tributes on social media. 

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Isiah Whitlock Jr., who played crooked politician Clay Davis on "The Wire," tweeted that Williams was "one of the nicest brothers on the planet with the biggest heart. An amazing actor and soul."

Actor John Cusack tweeted that his portrayal of Little was "among the greatest performances tv and film has ever seen."

Another "Wire" costar, Wendell Pierce, wrote, "The depth of my love for this brother, can only be matched by the depth of my pain learning of his loss. An immensely talented man with the ability to give voice to the human condition portraying the lives of those whose humanity is seldom elevated until he sings their truth."

New York City police say Michael K. Williams was found dead Monday, Sept. 6, 2021, at his apartment in Brooklyn. He was 54.  (Evan Agostini/Invision/AP, File)

Director Ava DuVernay wrote in an Instagram post, "I remember the times you’d come on set even when you weren’t on the call sheet… Just to share a hug. To cheer us on. Strolling in like the King that you were. For just a flash to give some love - then gone."

Riz Ahmed, who co-starred in the miniseries "The Night Of" with Williams penned, "Rest in Peace MKW…rare talent, unique soul. So generous, humble, bada--, and so unapologetically yourself. Thank you for the art you gave us & all you taught me."

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Joel McHale, who co-starred with Williams on the comedy series "Community," tweeted, "Damn. Just damn. Mr. Williams was such a Good, kind, wonderful man. He was so funny on Community and, of course, an actor for the ages."

Jeffrey Dean Morgan posted, "Goddamn it. We lost one of our best, most talented. RIP #MichaelKWilliams. Breaks my heart."

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Williams was known for roles on "The Wire," "Lovecraft Country," and "The Night Of." (Rodin Eckenroth/FilmMagic)

James Gunn said on Instagram, "I met Michael K Williams many years ago while he was still on The Wire. He was then and has been since then, one of my favorite living actors. In addition he was one of the sweetest, most gentle souls I’ve ever met. Soft-spoken and easy to laugh, so incredibly different from many of the characters he portrayed. His passing today is heartbreaking. Today I mourn his loss and send my love to all the others in this world who loved him. Rest In Peace, Michael."

Ellen Barkin, who co-starred with Williams on "Brooklyn’s Finest," said, "We lost a great talent. What he brought was his and only his. Rest In Peace Michael K Williams."

Director Spike Lee admitted he was "shook" over Williams' death. He paid tribute on Instagram, "Whew Lawd,Dis Is A Tough Day-1st We Lose The Great French Actor JEAN PAUL BELMONDO And Now My Brother MICHAEL K. WILLIAMS. Our Brother Passed Away Today In His Home,Da People’s Republic Of Brooklyn. I’m Shook. Drinking A Bottle Of Italian Red Wine-Brunello Di Montalcino By Myself. Our Greats Are Leaving Us Left And Right. GOD BLESS."

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Williams was also a ubiquitous character actor in other shows and films for more than two decades, creating another classic character as Chalky White in HBO's "Boardwalk Empire" from 2010 to 2014, and appearing in the films "12 Years a Slave" and "Assassin’s Creed." 

He is up for an Emmy for his role in HBO's "Lovecraft Country." A win at the Sept. 19 ceremony would be his first in four nominations.

The Associated Press contributed to this report.