As his murder trial began in December 2009, Arvel Marshall made a desperate plea to his lawyer: Ask for surveillance videos of the Brooklyn street where he was accused of fatally shooting a 22-year-old man.

Mr. Marshall’s lawyer made a halfhearted request for the footage. The prosecutor said initially that it did not exist, then that it was not accessible, and the judge moved the trial along. Mr. Marshall was convicted and sentenced to 25 years to life in prison.

But on Friday, after Mr. Marshall had been incarcerated for 16 years, prosecutors with the Brooklyn district attorney’s office said he had been right all along and asked the court to overturn his conviction.

The video in question showed two men, neither of them Mr. Marshall, walking down a Crown Heights street toward where Moustapha Oumaria was sitting outside his home with three friends.

One of the two, whose clothing matched a witness’s description, is seen “removing an object from his waistband, handling the object by his right hip,” prosecutors said. Shortly after Mr. Oumaria was shot in the head, the same men run in the opposite direction, the footage shows.

“Everyone involved in this case — defense, prosecution, police, and the court — failed, depriving Mr. Marshall of a fair trial,” said Eric Gonzalez, the Brooklyn district attorney. “A critical piece of evidence was not turned over, leading to this unjust conviction.”