BOSTON -- Boston Police said Wednesday they have arrested three more people in the marathon bombing investigation, but gave no additional details.
Sources told NBC News the three are college students who are suspected of removing items from the dorm room of Dzokhar Tsarnaev, 19, who is charged with placing a bomb at the finish line of the April 15 race.
It was unclear if the persons in custody had any knowledge or involvement in the bombing or its aftermath.
"Please be advised that there is not threat to public safety," the Boston Police Department said on its website.
Authorities said Tsarnaev and his older brother Tamerlan carried out the blasts that killed three and wounded more than 200.
Tamerlan Tsarnaev, 26, was killed during a firefight with police. Dzhokhar, 19, was arrested after a manhunt and has been charged with using a weapon of mass destruction.
Law enforcement officials have told NBC News that Dzhokhar told them during questioning he and his brother wanted to defend Islam after the American wars in Iraq and Afghanistan.
The brothers, ethnic Chechens who had been living in the U.S. for more than a decade, were seen on video at the marathon. Investigators have been trying to determine if they received assistance from anyone else in the U.S. or abroad.