Authorities in Boston are investigating a Boston Marathon picture which shows a man walking on a rooftop overlooking the finish line during one of the blasts.
The chilling image appears to show a male figure striding across the roof of a high rise block as an explosion rocks the marathon finish line directly below.
The man has not been identified, fueling speculation online that the mystery figure watched the carnage unfold beneath him in Boston.
He was caught on camera from a distance by spectator Dan Lampariello at the precise moment the second bomb exploded several hundred yards from the first.
It is unclear whether the rooftop was watching the race or lived in the building he was stood on top of.
Within minutes of the picture emerging online, the topic 'Boston Marathon roof' was trending online.
One Twitter user wrote: 'The picture of the guy on the roof overlooking the second explosion is the spookiest thing I have ever seen.'
Police in Boston are said to be cracking down on rooftop parties in the city.
In 2011, a 22-year-old college student fell through a skylight while watching the Boston Marathon from the top of a Brookline apartment.