The page — though visible to any Facebook user before it vanished into the digital ether — appears to have drawn no public notice until an obscure criminal case in State Supreme Court in Brooklyn last month, the gun possession trial of an out-of-work Brooklyn food-service worker named Tyrone Johnson. His defense lawyers put many of the controversial remarks before the jury. But when that too seemed to draw little notice outside the courthouse, the lawyers, Benjamin Moore and Paul Lieberman of Brooklyn Defender Services, provided a digital copy of the Facebook conversation to The Times, saying it raised broad questions about police attitudes.

While preparing for the trial, Mr. Moore checked to see if the officer who had arrested his client, Sgt. Dustin Edwards, was on Facebook. He was. Mr. Moore noticed that Sergeant Edwards’s profile showed he belonged to a Facebook group formed, it said, for “N.Y.P.D. officers who are threatened by superiors and forced to be victims themselves by the violence of the West Indian Day massacre.”

The group’s title, “No More West Indian Day Detail,” attracted Mr. Moore’s attention because Sergeant Edwards had arrested Mr. Johnson in the predawn hours of the celebrations before the parade in 2010.

Mr. Moore said that when he clicked on the link — the page was apparently public — and began reading a conversation that ran 70 printed pages, he was struck by what seemed to be its reckless explicitness. “I found it astounding,” he said. He made a digital copy. When he looked two days later, all trace of the group was gone.

At the trial, the defense lawyers argued that the gun Sergeant Edwards said he found near their client had not belonged to Mr. Johnson. Mr. Johnson is black and lived in the parade area. The defense suggested that Sergeant Edwards might have planted the gun.

Sergeant Edwards testified he had never posted a comment on the group that protested the West Indian Day detail. He said his involvement had amounted to nothing more than clicking on the name of the group that included “a lot of the people in another police group that I’m in.”

Still, through Mr. Moore’s questions, Justice Bruce M. Balter’s courtroom got an earful of what Mr. Moore described as the bias-riddled police commentary.