The Boston Police Department broke the news on Wednesday.
The AP later named two of the three suspects as Azamat Tazhayakov and Dias Kadyrbayev. Reuters reports they are both Kazakhstan citizens and classmates of Dzhokhar Tsarnaev.
Two of the three people taken into custody on Wednesday in the investigation of the Boston Marathon bombings were college classmates of suspect Dzhokhar Tsarnaev who may have thrown away a knapsack at Tsarnaev's request, a law enforcement source said.
Both are citizens of Kazakhstan who previously had been held by U.S. immigration officials on suspicion of violating terms of their visas, said the source, who was briefed on the investigation but unauthorized to be identified publicly because the information was under court seal.
The third person taken into custody on Wednesday was a U.S. citizen, and all three were being investigated for actions taken after the April 15 bombings, the source said.
Boston Police released another statement to clarify there is no threat to public safety:
Please be advidsed that there is not threat to public safety. Three additional suspects have been taken into custody in connection to the attack on the Boston Marathon. Aside from the aforementioned, there is no additional info to release at this time. Additional details will be provided when they become available. No info available at this time relative to a press conference.