Video and photographs taken at the scene have led investigators to a number of 'people of interest' but in particular one young man wearing a baseball cap and seen carrying a bag and then dropping it off at the site of one of the explosions.
He is also seen talking on a mobile phone and investigators are now tracing those phone records to see if they can identify him.
They are also using computer facial recognition technology to see if his face matches any face on criminal records.
They are scouring rooftops, scouring parks we saw yesterday an inch by inch, painstaking surge of a huge crime scene around around.
But they are talking about a significant breakthrough.
People in this city still on edge still hurting.
New photos of the suspects identified by officials as Dzhokar A. Tsarnaev, 19 and Tamerlan Tsarnaev, 26 have been released.
The FBI has released pictures and video of two suspects in the Boston Marathon bombing, seeking the public's help.
President Obama attended a memorial service in Boston today, paying tribute to the victims of Monday's terrorist attack.