Murtagh today is unrecognizable from the news photos of him on the day of the verdict 33 years ago — a skinny nebbish in front of microphones, dressed in the unflattering florid polyester of the time. It was only his second trial ever. (He’d lost the first, against a distributor of dirty magazines.) During the final tense mornings of the trial, Murtagh would wake up, then throw up.

Responsibility for the huge case had fallen into his inexperienced hands by default in 1975, when the original lead prosecutor, Victor Worheide, died suddenly of a heart attack.

Murtagh has had an unconventional career: Half his professional life, he estimates, has been consumed by two cases only: the prosecution of Jeffrey MacDonald and the prosecution of the Libyan intelligence officer who masterminded the explosion of Pan Am Flight 103 over Lockerbie, Scotland. He won them both.

Murtagh and I are in his living room. He’s remembering how he first got involved with the MacDonald case, back in 1971, as an Army lawyer. A police investigator was showing him the crime-scene photos, trying to urge him to join the prosecution. They got to a certain picture, the one he and I are looking at right now.

Because of a strange coincidence — each member of MacDonald’s family had a different blood type — police could easily map who had been attacked, and where. The tale of the blood was very different from the tale MacDonald told. There were many significant discrepancies, among them that the doctor’s pajama top was stained with Colette’s blood before the garment was torn. That should have been impossible if, as MacDonald said, it was torn in the initial struggle with the intruders.

“A Wilderness of Error” doesn’t dwell long on the blood evidence. Citing shoddy detective work — much of the initial police work was, indeed, sloppy — Errol Morris basically dismisses all of it as tainted. Outside the courthouse in Wilmington, he tells a camera crew that he doesn’t see “a shred of evidence” suggesting MacDonald’s guilt.

It was an odd choice of language, considering. Shreds of MacDonald’s torn pajama top were central to the case against him: Broken pajama threads were not found in places they should have been if his story were true — near the sofa on which he was allegedly attacked, for example — but were found in places they should not have been, such as beneath Colette’s body.

And then there was Murtagh’s smoking gun — his big triumph at trial.

McDonald had told police that after he woke up, he put his pajama top on Colette’s chest. Forensics showed that 21 ice pick holes in Colette’s body lined up perfectly with 48 holes in Jeffrey MacDonald’s pajama top, when the pajama top was folded a certain way so that some punctures went through more than one layer of fabric. Murtagh argued that the only explanation for this was that MacDonald had delivered those blows himself to a dead or dying Colette. The jury bought it. But “A Wilderness of Error” finds this preposterous — the prosecution never established that this was the only way the pajama could have been folded to get that pattern of holes, Morris argues; conceivably, you could manipulate any piece of cloth into innumerable shapes that would produce the same pattern.