The photo -- taken by spectator Dan Lampariello -- shows the explosion of the second bomb hundreds of yards from where the first explosion took place, Yahoo! News reports.
Not too long after the photos were posted on Twitter, several users began to notice a man at the top of a building who wasn't reacting to the explosions.
"Who's that guy on the roof?? #BostonMarathon #PrayforBoston," tweeted a parody account for musician Frank Ocean.
"I wonder if the FBI could look at cameras from that building to find the guy on the roof in the top left ... #Boston," another user tweeted.
"If that guy is the murderer or not on the roof that is still a pretty sick photo. As in creepy," wrote another.
Boston Police said Monday evening they had no suspects is custody.
Richard DesLauriers, the special agent in charge of the FBI's Boston office, said his agency would lead the government investigation requested by President Barack Obama.
"It is a criminal investigation that is a potential terrorist investigation," DesLauriers said.
Speaking at a news conference on Monday afternoon, President Obama promised those responsible for the bombings would feel the "full weight of justice."
"We still do not know who did this or why, and people should not jump to conclusions before we have all the facts," Obama said. "But we will find out who did this and we'll find out why they did this. The responsible individuals and groups will feel the full weight of justice."
As of now three people have been declared dead and 144 have been declared injured by the explosions near the Boston Marathon.
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