The police said they had no suspects in the custody while White House sources said there was no immediate claim of responsibility behind the attacks.
TV reports said later in the day that law enforcement officers were searching an apartment in the Boston suburb of Revere in connection to the blasts. Massachusetts State Police said a search warrant related to the probe into the blasts was served on Monday night in Revere.
It was also reported that federal investigators and law enforcement personnel were intensively checking all outgoing international flights from the USA, indicating that they could be looking into a foreign connection to the attacks.
The heavy volume of mobile phone calls around downtown Boston after the blasts also disrupted the cell service, stopping many from reaching to people on the spot, said The Boston Globe.