As the horror unfolded in Newtown, Conn., last Friday, dozens of news sources reported a striking element of the story: that Nancy Lanza, mother of shooter Adam Lanza, was a teacher at the school where her son killed 26 people before killing himself.

This account persisted — in news reporting and in conversation — and seemed to fill in a critical element of the tragedy: a motive.

As it turns out, she was not a teacher. Nor does it appear that Nancy Lanza had been a substitute or a teacher’s aide at Sandy Hook Elementary School, as some reported. Other accounts said incorrectly that she had been killed at the school along with her students, rather than at home before her son’s rampage at the school.

While it was a relatively minor error in a string of inaccuracies about the Newtown shootings, the Nancy-Lanza-was-a-teacher story is an important tale in its own right.

As the story made the rounds, TV commentators speculated that Adam Lanza acted out of rage toward his mother and had transferred his murderous impulses toward the innocent lives under her care. “When you think about the details of the crime, he began by shooting his mother in the face, taking her weapon and then destroying everything precious to her, her colleagues and her children, and then killing himself,” Charles Krauthammer, a syndicated columnist with The Washington Post, said on Fox News.

On Sunday night, President Obama spoke at a vigil for those who died in the Newtown, Conn., tragic shooting at the Sandy Hook elementary school. (The Washington Post)

The error appears to have originated Fridaywith a carefully hedged Associated Press report about the shootings just after noon, about three hours after the first reports of shots being fired at the school.

“At least one parent said Lanza’s mother was a substitute teacher at the school,” the wire service said. “But her name did not appear on a staff list. And the official said investigators were unable to establish any connection so far between her and the school.”

The story began to spread. Just after 2 p.m., CBS reported that Lanza’s mother was a teacher and that many of the victims were her students. CNN reporter Susan Candiotti identified her as a teacher, but she said she wasn’t certain where Lanza had died. The Washington Post and the New York Times also reported that she was a teacher.

An hour later, AP “confirmed” its earlier story. It cited “a law enforcement official” in Washington who said that the suspect was Ryan Lanza, that his younger brother was being held for questioning as “a possible second shooter” and that their mother taught at the school.

Although the misidentification of the suspect was corrected within an hour, the teacher angle wasn’t knocked down for almost 11 hours after it was first reported. At 11:16 p.m., AP moved an update, saying no connection had been found. Nevertheless, CNN and other news outlets continued to report the story as fact until early Saturday.

An AP spokesman, Paul Colford, said the news service got bad information from sources “we had no reason to disbelieve.” He added, “We were confident in our sources, and our sources were wrong.”

Yet the widespread reporting of the teacher story may have highlighted the news media’s tendency to fill in the blanks on initially confusing and tragic stories, says W. Joseph Campbell, a communications studies professor at American University.

The idea that Nancy Lanza was killed with her students “is a narrative that does hang together and explains the story in ways the real story doesn’t,” said Campbell, the author of “Getting It Wrong: Ten of the Greatest Misreported Stories in American Journalism,” a study of media-driven myths. “It’s hard for us to accept the idea that something so horrible was completely random. The idea that she had little or no connection to the school makes it harder to wrap your mind around such a horrific and senseless act.”

Campbell documented a similar phenomenon of “narrative fulfillment” in news coverage of the aftermath of Hurricane Katrina in 2005. Many accounts described rampant acts of violence and mayhem in New Orleans that turned out to be mostly false. Such stories, he said, were fed by the media’s assumption that New Orleans was populated by a lawless underclass given to such behavior. Foreign press outlets, he said, were quick to add a layer of anti-Americanism, suggesting that the alleged chaos reflected the decline of American power.

While the media have made errors in many other breaking-news situations, the number of errors that grew out of the events in Newtown suggests “we’re dealing with a new normal in terms of what happens in major events,” said Craig Silverman, who writes Regret the Error, a blog about reporting mistakes.

Constant deadlines, intense competition, reduced news staffs and instantaneous transmission via social networks, Silverman says, make it likely that it will happen again. “People have to realize that this is going to happen a lot,” he said.

Given that it’s unlikely that competition for news will somehow slow down, he advises the media to find “constructive ways” to be as transparent as possible with readers and viewers. “We are our own worst enemies,” he said. “We should tell people what we’re not ready to report and why. Not everything is rock solid. We should tell them how solid what we know is.”