A Sept. 11 mystery 13 years in the making has — through the collective power of Facebook, Twitter and country music’s Blake Shelton — finally been solved.

Every year since the attacks, Elizabeth Stringer Keefe has attempted to identify the people in a wedding photo one of her friends found in the debris near ground zero. The image, faded and torn in places, seemed to have come from someone’s desk — and the fate of both owner and subjects could only be guessed at. So Keefe paged through photos of the deceased, looking for someone from the picture. She repeatedly posted the image to Twitter and Facebook, begging anyone with information to respond.

On Friday, someone finally did: His name was Fred Mahe, and in the image, he’s the gentleman on the left.

Mahe worked on the 77th floor of the second World Trade Center tower, but was not in the building when it collapsed. Neither was any member of the wedding party, in fact: Everyone in the picture is still alive. The bride and groom, Christine and Christian Loredo, live outside of San Francisco and have a 6-year-old son.

“I think it’s nice to know that people out there care so much for strangers,” Christine, the bride, told ABC News over the weekend. “It gives me confidence in humanity.”

But, with no offense intended to Keefe (whose persistence was, honestly, both heart-warming and remarkable), maybe it should give her confidence in network theory, too. After all, there’s a reason the photo went viral this year, when it didn’t go viral all the long years before. For the first time, it hit what information scientists call the epidemic threshold — the point at which a thing reaches enough nodes in the network that it can’t easily die out.

To be clear, network theory is really complex and mathematical, and I’m simplifying here. But when information moves through a social network, it doesn’t move at random: The information pings from one “node,” or person, to another, and the relationships between all those thousands of nodes create an intricate geography of influence and power. When we refer to someone as a “power user” on a platform like Twitter, it means that they’re a very influential node, or connector, in the network. And when we say something goes “viral,” we actually mean that its spread through the network mirrors the spread of a disease: fast, multi-nodal and self-replicating.

Keefe’s photo never “went viral” before, because it never hit the epidemic threshold. It never reached enough nodes outside of her immediate network.

But when the image was shared first by a popular Boston news account, @universalhub, and then by Blake Shelton, that changed. This is the message Shelton retweeted:

The story was promptly picked up by outlets like Buzzfeed and Gothamist, both of whom cover viral media. A friend of Frank Mahe read Gothamist. And just like that, a mysterious photo was reunited with its owner, courtesy one very persistent professor … and a country music star.

Keefe is now enjoying the completion of her decade-long quest. She and Mahe met in New York this morning; she tweeted that she’d be returning the photo to him, after its 13 years hidden in a novel on her bookshelf. Shortly before 8 a.m., she posted another photo to Twitter: a selfie with Mahe, both of them grinning into the camera.