MSNBC host Lawrence O’Donnell questioned on Tuesday why St. Louis County prosecutor Bob McCulloch would rely so heavily on testimony from one witness who seemingly changed his story for the grand jury that ultimately decided not to indict police officer Darren Wilson for killing 18-year-old Michael Brown this past August.

“We would like to be able to judge the credibility of all the witnesses. We would all like to be able to do that,” O’Donnell said. “We would like to know as much as we can about them. But what we know about ‘Witness #10′ is that the prosecutors willfully did absolutely nothing to help the grand jurors judge the reliability of the only witness who completely agreed with Darren Wilson’s description of what Michael Brown’s movements were.”

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O’Donnell referred to the witness as McCulloch’s “favorite witness,” saying that McCulloch had enough trust in the witness’ observations to highlight only his testimony while announcing on Monday that Wilson would not be indicted.

But during his testimony, O’Donnell said, the witness said he was not sure what kind of “body gesture” Brown made before being shot and killed by Wilson on Aug. 9, and that he could not “fully recall” what Brown did, except to say Brown was not surrendering.

“There’s the district attorney’s favorite witness — the only one he quoted last night — saying, ‘I cannot fully recall, I’m not sure, I’m not sure,’ within the body of an answer in which the only thing he’s absolutely sure of is that Michael Brown did not do a surrendering motion,” O’Donnell said.

In a real courtroom, O’Donnell said, that kind of answer would not survive cross-examination.

Instead, he explained, the only real challenge the witness received from prosecutors was to be asked if he wore glasses or contacts after describing his vision as “pretty good.” And even then, O’Donnell said, the witness was not asked when his eyesight had last been tested, or if it had been tested at all, after telling police two days after the shooting that he saw the altercation between Brown and Wilson from 100 yards away.

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“Stop,” O’Donnell said. “Think about that. There’s a football field with the referee on one goal line. And the action he is judging, 100 yards away, at the other goal line. Who among you would trust the judgment of that referee standing on one goal line 100 yards away from the action at the other goal line?”

If McCulloch had ever watched a football game, O’Donnell argued, he would not put that much faith in an official in that situation. But six weeks after making that statement to police, the witness told the grand jury he was “50 to 75 yards” from the encounter between Brown and Wilson.

“He cut the distance in half, to 50 yards,” O’Donnell said. “He did that under oath before a grand jury who had no idea that the last time he was asked that question, two days after the shooting, he said double that. And no prosecutor in that grand jury room put his police interview transcript in front of him and asked him if he remembered saying 100 yards, instead of 50 yards.”

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Watch O’Donnell’s commentary, as aired on Tuesday, below.