“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.