Mr. Mader tried to calm him down. But two more officers arrived on the scene within minutes, and one of them, upon seeing Mr. Williams waving the weapon, shot him dead.
Mr. Williams’s pistol was not loaded, it was later reported.
State police investigated the episode, and Mr. Mader, a former United States Marine who had served in Afghanistan, received notice of his termination on June 7. The notice cited his “apparent difficulties in critical incident reasoning.”
At a news conference one day later, a county prosecutor announced that the officer who had shot Mr. Williams was justified in doing so.
“Rather than respect Mr. Mader’s informed judgment and experience and his reasonable attempt to de-escalate the situation,” the A.C.L.U. complaint argued, “the City of Weirton, in a flawed effort to buttress the other officer’s use of deadly force, wrongfully terminated Mr. Mader’s employment.”
Mr. Mader had been with the Weirton police for only 11 months and was still a probationary employee when he was fired. His termination letter did not focus solely on the May 6 episode; instead, it said he had failed “to meet probationary standards of an officer.”
But Joseph Cohen, executive director of the A.C.L.U. of West Virginia, said the police were motivated in large part by Mr. Mader’s decision not to shoot Mr. Williams, and rushed the officer out of his position.
Mr. Mader could not be reached for comment on Thursday, but told The Pittsburgh Post-Gazette, a Pennsylvania newspaper, that he did not have an opportunity to thoroughly explain his side of the story. “I thought I was going to be able to talk to him and de-escalate it,” he said, referring to his confrontation with Mr. Williams.
The Weirton police department did not return calls or respond to an email seeking comment.
Stories of white officers shooting black men and boys — including Michael Brown in Ferguson, Mo., and Tamir Rice in Cleveland, Ohio, in 2014; Walter Scott in North Charleston, S.C., in 2015; Alton Sterling in Baton Rouge, La., in 2016; and Jordan Edwards near Dallas, Tex., last month — have spurred protests across the country and increased public scrutiny of policing practices.
The most important thing in the case of Mr. Williams, Mr. Cohen said, “is that a young African-American man was killed.” But he added that Mr. Mader’s decision not to pull the trigger — and his subsequent termination — presented a unique legal opportunity to dig deeper into the issue.
“We took the case because it is important for what it says about policing in the U.S., what it says about policing in West Virginia and what it says about policing in small towns,” Mr. Cohen said. Court proceedings, he added, could go on for years.
On Thursday, both the Weirton city manager and the lawyer representing the city, Cy Hill, said they had not yet received the A.C.L.U. complaint. “When we do get a copy, it will be reviewed by legal counsel and then a determination will be made as to whether we’ll be making any comments on this particular lawsuit,” the city said in an emailed statement.
Months before his death, Mr. Williams’s name was in a newspaper for a happier reason. His son was the first baby born at the Trinity Medical Center in Steubenville, Ohio, in 2016.
“I am very happy that he is healthy,” Mr. Williams said at the time to The Herald-Star, a local newspaper. “He is starting the New Year off right for all of us. Now we are giving him a lot of love to welcome him into the world.”
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