But if you think you haven’t seen it, you are right. In fact, it may not exist at all.

The public first heard about the video from then-Massachusetts Gov. Deval Patrick on “Meet the Press” on April 21, two days after Tamerland Tsarnaev died following a shootout with police and Dzhokhar Tsarnaev was captured. Host David Gregory posed this question: “Is there anything on the videotape that maybe the public hasn’t seen about his reaction that was particularly telling that moved the investigation along?”

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Patrick immediately said he had not seen the videotape. But, he said, “it does seem to – to be pretty clear that – that this suspect took the backpack off, put it down, did not react when the first explosion went off, and, and then moved away from the backpack in time for the second explosion. So pretty, pretty clear about his, his involvement and pretty chilling, frankly, as it was described to me.”

The videotape made its second appearance just two days later, also on NBC, in a report on the criminal complaint against Tsarnaev. NBC said the complaint “is built on surveillance video, which shows him dropping a backpack near the finish line and walking away as the bombs go off.” And then it showed the video — sort of.

The report began with what appears to be actual surveillance video the FBI used to pinpoint the Tsarnaevs. They are shown turning onto Boylston Street and walking, Dzhokhar carrying a backpack easily on his right shoulder. “Seven minutes later,” says correspondent Anne Thompson, “Dzhokhar Tsarnaev drops his knapsack at the metal barrier in front of the Forum restaurant.” By this time, though, we are looking not at a video but at a still photograph that shows the backpack on the ground and Dzhokar Tsarnaev a few feet away, apparently walking away. Then again, it could be a picture of him just walking by. The moment of his setting down the backpack is narrated but not shown.

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Few viewers could be expected to notice the difference, though, and even fewer would remember it. Last month, during a hearing on the defense’s latest bid to have the trial moved, appeals Judge Juan Torruella made reference to the video of the defendant setting down the backpack, saying it had been widely broadcast. Prosecutor William Weinreb, whom Torruella was addressing, didn’t contradict him. However, in his dissenting opinion last week, arguing against the court’s decision to deny change of venue, Torruella mentioned other coverage but not that video.

But the day after the appeals court hearing, a prospective juror said she couldn’t get that video out of her mind, and here was how she described it: “The image of him putting the backpack behind that little boy.” Even in the still shown by NBC, no little boy is visible. (The judge had ordered prospective jurors not to read, watch, or listen to anything about the case.)

The Associated Press’s Boston reporter used that juror to headline that day’s dispatch from court, saying that the juror was “describing an image authorities say shows Tsarnaev placing a bomb near 8-year-old Martin Richard, one of three people killed when twin bombs exploded near the marathon finish line April 15, 2013.” The story ran all over the country. Even Boston’s public radio station WBUR, whose own reporters have been supremely skeptical of the investigation, ran it and promoted it uncritically.

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Anyone who is in the business of eliciting people’s memories about historic events knows that collective memory has a way of displacing personal experience. Maria Konnikova recently wrote in The New Yorker about psychological studies that show that people typically misremember what, when and how they learned about big things that happened just a couple of years earlier. Oles Adamovich and Daniil Granin, two Russian writers who compiled an oral history of the Siege of Leningrad, recalled that their subjects would spend hours reproducing received memories — stock stories that they had read in books or newspapers or seen in films — before getting to their own — if they ever did.

In the Tsarnaev trial, the defense has tried at least half a dozen ways to make the very point that saturation publicity has altered people’s perceptions in ways of which they are not necessarily aware – and that this renders meaningless jurors’ sincere commitment to being impartial. After imagining this video for a couple of years, what will they feel when they finally see it during trial?

But now an even more difficult question has been raised: What if jurors never see this video? What if there is, in fact, no video? That’s just what defense attorney David Bruck said Monday during a hearing on final pretrial motions: The video doesn’t exist.

If it existed, prosecutors would use it as evidence. If they were going to do that, they would have had to turn it over to the defense a long time ago. Apparently, they have not.