His disappearance mystified the authorities and his family, who said they had been in daily communication with him before he left.

Last Thursday, shortly after the F.B.I. released grainy surveillance photographs of the two men they believed were the Boston bombers, some Internet users began to speculate that Mr. Tripathi might be one of them. They began posting messages suggesting as much on the Facebook page the family used to raise awareness about their search for Mr. Tripathi. As the policed pursued the two suspects on a fiery chase through the Boston area, the rumor spread, leading news vans to stake out the Tripathis’s family home in Bryn Mawr, Pa., and reporters to make dozens of calls to the Tripathi family’s cellphones.

“It was absolutely horrible,” said Ms. Tripathi, who said her family knew immediately that neither of the men in the photographs was her son — a man who they said literally would not hurt a fly, choosing instead to set insects free outside.

By last Friday morning, investigators had released the suspects’ names — Tamerlan and Dzhokhar Tsarnaev — absolving Mr. Tripathi of any connection to the case. Apologies began to pour in to the Tripathi family — including one from the online forum Reddit, where users had doggedly pursued the rumor — who began a new social media campaign they hoped would persuade Sunil to come home.