The plot thickens?  Three college friends of Boston Marathon bombing suspect Dzhokhar Tsarnaev are under arrest, suspected of removing items from his dorm room after the April 15 attack, sources said Wednesday.

KBJR-TV, our local NBC News affiliate, is reporting that two of the pals were detained April 20 on immigration charges and a third has now been taken into custody, sources said. They are expected to face obstruction of justice charges, the sources said.

There was no indication the three University of Massachusetts-Dartmouth students had any prior knowledge of the bombing.

"Please be advised that there is no threat to public safety," the Boston Police Department said on its website, confirming the arrests but releasing no further information.

Authorities said Dzhokhar Tsarnaev, 19, and his older brother Tamerlan carried out the blasts that killed three and wounded more than 200 near the finish line of the race.