The showdown with Mr. Kalven was prompted by Mr. Van Dyke’s lawyer, who suggested that Mr. Kalven obtained leaked documents and may have passed along that information to witnesses of the shooting, influencing their accounts to investigators.

The fight over Mr. Kalven’s sources threatens to push the start of the trial even further down the road, delaying legal proceedings that many Chicagoans say should have already begun. Mr. Kalven, the founder of the Invisible Institute, a local independent news organization, said he is prepared to fight what he sees as a broadside on his First Amendment rights. In a legal filing this month, his lawyer called the effort an “unjustified fishing expedition.” Mr. Kalven said in an interview that he is willing to testify — but not about his sources of information.

“The one thing that I’m clear about is that I’m not revealing my sources,” said Mr. Kalven, sitting in his office on the South Side this month as silver commuter trains swooshed by in the background. “If we’re going to have a full-blown First Amendment controversy over this, it’s going to take months or years.”

Daniel Herbert, Mr. Van Dyke’s lawyer and a former police officer, declined to be interviewed, citing a gag order imposed by Judge Vincent Gaughan of the Cook County Circuit Court. But Mr. Herbert’s efforts suggest that he may try to urge the judge to bar witnesses to the shooting from testifying at trial, on the grounds that their recollections were tainted by information Mr. Kalven passed along to them.

During a hearing in October, Judge Gaughan noted a suggestion that Mr. Kalven may have gotten the name of a shooting witness from someone connected to the Independent Police Review Authority, an agency that, at the time of the shooting, was responsible for investigating claims of misconduct and excessive force by Chicago police officers. “He will be testifying,” the judge said of Mr. Kalven, according to The Chicago Sun-Times.

Mr. Kalven, 69, says he is bewildered to suddenly become a player — and no longer an observer — in the case against Mr. Van Dyke.

It has been three years since he first heard the name Laquan McDonald. The news of the teenager’s death on the Southwest Side of Chicago on the night of Oct. 20, 2014, received modest coverage in local news media. An article the next day in The Chicago Tribune, citing a police union spokesman, noted that Mr. McDonald was armed with a knife, behaving erratically. The report said that he had “allegedly lunged at police” before being fatally shot by an officer.