LOWELL – As criminal cases go, it was a depressingly common trial, hardly the stuff of sensational headlines. A man was accused of beating and trying to strangle his wife.

But for the jurors chosen to sit in judgment of the accused 56-year-old in the old 19th-century courthouse here on Gorham Street late last month, it was the center of their universe for three days.

They had a sworn duty. They held a man’s fate in their hands.

They heard the evidence. They reviewed the exhibits. They listened to legalisms from the attorneys and from the judge.

And then, locked in a well-worn jury room, they rendered a guilty verdict that now haunts one of them.

“I feel so bad about this decision,’’ juror Rob Moir told me last week. “This process just went haywire.’’

In a letter he sent to Superior Court Judge Robert L. Ullmann, Moir was even more explicit.

“I am ashamed that my desire to be part of a group impaired my judgment,’’ he wrote. “I greatly regret my irresponsible actions that resulted in an innocent man being found guilty of something he did not do.’’

What happened in the jury room of the rambling courthouse here is a cautionary tale about the duties and responsibilities of jurors, who take an oath to follow the law and the evidence.

I have reviewed the lengthy file of the case of the Commonwealth versus this defendant. But I did not sit through the testimony. There was no trial transcript available to review. For all I know, Moir’s fellow jurors got it exactly right.

But what is clear is this: Justice requires a unanimous verdict. And Rob Moir now wishes he could go back in time. He wishes he stood his ground. Instead, he said, he was caught up in the socio-dynamics of that jury room.

And now he feels something that the defendant will reckon with at his sentencing on Friday: Guilt.

If you’re looking for a black-and-white tale here, you’ve wandered into the wrong column.

Briefly, here is the Commonwealth’s statement of the case:

On Feb. 6, 2016, the man appeared at the home of his wife on Nesmith Street here. He had just been released from the house of correction in Billerica, where he had been since August 2015, incarcerated for assaulting her. He showed up unannounced. His wife was unaware he was being released. There was an argument. It turned physical. Prosecutors said the man grabbed his wife by the throat and started to strangle her with one hand. She had visible redness on her neck and minor bruising on her face. She struggled to break free, which she did with the help of another man who was at the home.

As testimony unfolded, Moir and his fellow jurors reflexively obeyed the judge’s admonition to not talk about the case until the trial’s conclusion.

Instead, they discussed their courthouse commutes. One woman was selling her home and in the midst of a move. Moir and some other jurors brought coffee and doughnuts for their fellow jurors to share.

“Totally collegial,’’ said Moir, a 63-year-old father of three who runs an environmental advocacy group in Cambridge. He was the second to the last juror selected. He took notes during the trial. He took his duty seriously.

But as the trial unfolded, some things didn’t add up to Moir. And he carried those doubts with him. Was the accused strong enough to lift his wife up with one hand and strangle her? He didn’t think so, and neither did other jurors. The defendant, who is not being named here to protect the identity of the woman, was found not guilty on that charge.

That left the assault and battery charge.

When the jury foreman called for a vote, everybody raised their hand except for Moir.

He recalled this jury room exchange: “They say, ‘Rob! He’s guilty!’ And I said, ‘No, he’s innocent until proven guilty. It hasn’t been proven that he did this.’’

What followed was an examination of the victim’s bruises and how they might have been inflicted. “I’m saying, ‘He just pushed her off and that’s not assault and battery. She came at him. That’s justified.’ And this really chummy guy goes at me, saying, ‘Rob, he pushed her as hard as he could. That makes it assault and battery.’ And they’re all nodding, saying, ‘Yeah.’ ’’

And then Moir, to his astonishment, suddenly was with them.

“We all raised our hands and went, ‘Yaay!’ And it’s like: Oh my god, I just (convicted) this guy and I’m thinking, well, the judge will work it out. That’s his problem.’’

Except, mostly, that’s not how it works.

Juror regret is generally not enough for a retrial, or even a hearing before a judge. There are plenty of examples of juries who declare themselves hopelessly deadlocked and, then, late on a Friday afternoon they come back with a unanimous verdict.

Deliberations are organic things. They take on a life of their own. And sometimes justice is not as clean and clear-cut as prime-time television makes it out to be.

“There is a certain cascade effect in the jury room,’’ said Edward Schwartz, a jury consultant with DecisionQuest, a Waltham-based litigation consulting firm.

“You want to be part of the winning team,’’ he said. “The ugly underbelly of that is that when people disagree with the majority sentiment, sometimes that opinion is met with derision, contempt, hostility. If you think you’re a minority of one, it’s a pretty daunting prospect to go out on a limb if you’re the only one who feels that way.’’

Schwartz said what happened with Moir happens more often than many people would believe.

When I told the defendant’s attorney, Sharon Sullivan, about Moir’s remorse, she said this is a first for her.

“It hasn’t happened to me before,’’ said Sullivan, who said she intends to appeal the conviction, something she does routinely.

She was pleased that Moir had sent his letter to Judge Ullmann.

Through a court spokeswoman, the judge confirmed that he received the letter late last week. He did not wish to comment about it, the spokesman said.

Pamela Wood, who has been the state’s jury commissioner for 14 years, said the power entrusted to jurors is a hallowed one.

“We hope and expect that all citizens who are called for jury service take the obligation and the responsibility very seriously and they understand the import of what they’re called upon to do,’’ Wood said.

The Middlesex district attorney’s office, which prosecuted the case, declined to comment.

Moir said he is distraught and traumatized by his behavior.

“To me, I’m just shocked that I would step away from doing the right thing because of the need to be part of a cohesive group,’’ he told me the other day in his office overlooking Harvard Square. “I’m coming to terms with it. Part of the story is how a well-meaning juror can get distracted by social pressure and not do the right thing.’’

When the verdict was read in that old courtroom last month, Moir said he sat in the jury box. He avoided looking at the defendant he had just helped to convict. Instead, he kept his eyes on the judge.

And that’s who the defendant will be looking at again on Friday, when he is scheduled to be sentenced for the crime of which he was convicted.

That conviction was handed up by regular citizens like Rob Moir, the first-time juror who now feels guilty, too.

Thomas Farragher is a Globe columnist. He can be reached at thomas.farragher@globe.com.