Probably disguised as a clock in a briefcase.
But everyone knows that you only do clocks in a briefcase for science projects and not for violent acts....right?
On a side-note....did the kid in question bring his science-project to the White House to show the President? Hmmm....wonder....
According to Obama it could be a crock pot.
http://www.wcvb.com/news/obama-boston-marathon-bombings-2-brothers-and-crock-pot/36475830