The photo, taken by spectator Dan Lampariello, showed the second bomb explode several hundred yards from the first as runners approached.
Among the first Twitter users to spot the mysterious figure in Lampariello's photo was the operator of a parody account for R&B singer Frank Ocean:
The image was retweeted more than 2,000 times.
The U.K.'s Daily Mail pondered the figure on Twitter, too:
It wasn't long before the phrase "Boston Marathon roof," propelled by wild speculation, was trending on Twitter.
"Who's the guy on the roof?" one Twitter user asked. "He's not reacting to the detonation."
"The picture of the guy on the roof overlooking the second explosion is the spookiest thing I have ever seen," another wrote.
User @anon15576, an apparent member of the Anonymous hacktivist collective, tweeted: "#bostonmarathon bombing. #man on the roof. Anyone at all know #anything #fellow #anons plz tell me more about the man on the roof. #thanks."
It's worth noting that Boston police had been cracking down on rooftop parties along the marathon route.
In 2011, a 22-year-old college student fell through a skylight while watching the Boston Marathon from the top of a Brookline apartment.