A manhunt is under way for one of the two suspects in the Boston Marathon bombings after police sealed off a 20-block radius of a suburb where the suspect is believed to be hiding.

Photos: A law enforcement official checks his rifle during a search for the two men suspected of setting off two explosions during the Boston Marathon. Massachusetts State Police warned people in the suburb of Watertown not to open their doors. Credit: Lucas Jackson/Reuters

Police with guns drawn search for a suspect on April 19, 2013 in Watertown, Massachusetts. Earlier, a Massachusetts Institute of Technology campus police officer was shot and killed late Thursday night at the school’s campus in Cambridge. Credit: Mario Tama/Getty Images