UPDATED: April 16, 2013, at 3:15 a.m.

The tweets started circulating just before three in the afternoon.

Nicandro G. L. Iannacci ’13 was meeting with a professor at the John F. Kennedy School of Government when he checked his Twitter feed and saw the first mentions of an alleged bombing at the Boston Marathon.

Inesha N. Premaratne ’15 was in a government class when she scanned through her own feed and saw that several explosions had been reported near the race.

Isamar J. Vega ’13 did not even have a Twitter account when the day began, but she decided to set one up to follow the unfolding tragedy.

By Monday evening, the facts had been made clear: at least three were dead and more than 130 injured by two bombs detonated near the Boston Marathon finish line at Copley Square. But for much of the day, the truth was more opaque. With cell phone service in Boston down and TV news slow to sort out the details, members of the Harvard community turned to Twitter and other forms of social media to find and exchange the latest news about the bombings.

“Obviously there are upsides and downsides to this, but when you are lacking any kind of centralized information...it’s good to have an aggregate of different sources from people who were there,” said Kathleen E. French ’13, who was also very active on Twitter following the explosions.

But that absence of authoritative information—whether from law enforcement in Boston struggling to sort out the details of the bombings or from Harvard officials who remained quiet for several hours as unconfirmed bomb threats emerged around the Harvard Square area—allowed for a lot of misinformation, Twitter users on campus said.

From inflated death counts to a fake Boston Marathon Twitter account, it was at times difficult to sort through fact and fiction, students said.

“For me, to differentiate between information and misinformation was to see multiple authoritative people saying the same thing,” Iannacci said. “The danger of Twitter is one tweet going far if it’s not true.”

In particular, confusion over a dozen Cambridge Police Department tweets reporting potential threats, including several in the Harvard Square area, alarmed many students and faculty. All but one of those threats were investigated and cleared on Twitter by CPD as of press time early Tuesday morning.

Still, as police evacuated the Kennedy School and guards shut down vehicle gates into Harvard Yard, those following the reports online began to take precaution.

French said the CPD reports prompted her to stay indoors.

“I think that the Cambridge updates probably did freak people out,” she said. “It certainly scared me, but they were threats that the department was receiving, so it was not worth taking lightly.”

Iannacci agreed, saying that with a lot of misleading information circulating on Twitter, the police feed was the “ultimate authority.”

“I trusted the Cambridge Police Twitter,” Iannacci said. “To say that they created unsubstantiated fear is in some sense good, because they are doing their job and informing us as they learned that information.”

By late Monday, CPD tweeted its own response to critics of the reports.

“Tweets of INCIDENT/BOMBS/THREATS do not mean ppl making false reports. Means reporting what they see as suspicious & we respond 2 investigate,” the department wrote, adding in another tweet that the reports were triggered by unattended bags, clothing, and garbage.

While Harvard affiliates turned to Twitter in search of news after the bombing, they largely turned to other social networks to share the information they had. Facebook became a forum for Harvard affiliates to express grief and reassure friends and family members far from Cambridge that they were safe. Many House email lists featured long exchanges verifying the whereabouts and condition of various students who had been at the marathon. On its website, The Crimson compiled and published a list of University affiliates who had either run or attended the race and had been confirmed to be safe.

Within the broader Boston community, people turned to these and other platforms to offer news and resources. Google set up a site early in the day to help identify missing persons, and individuals throughout the area, including several Harvard affiliates, listed their information on a spreadsheet offering out-of-town runners places to stay for the night.

—Staff writer Nicholas P. Fandos can be reached nicholasfandos@college.harvard.edu. Follow him on Twitter @npfandos.