President Obama urges citizens to "be vigilant" as investigators search for those behind attack.
President Obama is calling the Boston explosions a clear act of "terrorism".
"Anytime bombs are used to target innocent civilians - it is an act of terror," Mr. Obama said Monday. "What we don't yet know, however, is who carried out this attack or why, whether it was planned and executed by a terrorist organization - foreign or domestic. Or was the an act of a malevolent individual."
Along the Washington D.C. Emancipation Day parade route police are on alert.
At the White House, Pennsylvania Avenue is once again closed to pedestrians.
Police have increased their presence at the Capitol Building entrances and other key locations.
Inside, lawmakers asked if the nation is doing enough to stop tragedies like yesterday's explosions in Boston.
"I think it's safe to say that, for many, the complacency that prevailed prior to September the 11th has actually returned. And so we are newly reminded that serious threats to our way of life remain," said Senate Minority Leader Mitch McConnell.
Just outside the Capitol Building was another reminder of the recent mass killings that have rocked the nation.
Survivors of the Virginia Tech shooting, six years ago today, remembered those who were lost.
In cities around the country, from Manhattan's financial district to the Los Angeles Airport, police looked for suspicious activity.
Counter terrorism experts from multiple agencies are teaming up to get answers.