By Matt Smith
(CNN) -- Two classmates of Boston Marathon bomb suspect Dzhokhar Tsarnaev and a third person face federal charges in connection with the April 15 attack, a federal law enforcement source said Wednesday.
Boston police announced the arrests Wednesday morning, adding that there was "no threat to the public." They were expected to appear before a federal judge Wednesday afternoon, U.S. government sources said.
The classmates -- Azamat Tazhayako and Dias Kadyrbayev -- are both from Kazakhstan and had been in federal custody on immigration charges already, their lawyers told CNN. The third person arrested is a U.S. citizen.
The Kazakh students face charges of making false statements to investigators and conspiracy to obstruct justice, according to a federal law enforcement source with firsthand knowledge of the investigation.
No details of the charges against the third person were immediately available. But Alan Dershowitz, a prominent defense attorney and Harvard law professor, said the new charges don't appear to be related to the bombing itself.
"If they knew about the bombing, if they were involved in the bombing, the charges would be conspiracy to do the acts for which the other man has already been charged," Dershowitz told CNN. "So it sounds like at this point in time the only evidence they have is actions that took place after the bombing."
Two bombs exploded in the crowd gathered near the finish line of the marathon, killing three people and wounding more than 260. Federal agents blame Dzhokhar Tsarnaev and his brother, Tamerlan, of carrying out the attack.
Tamerlan Tsarnaev died after a shootout with police on April 19, while Dzhokhar Tsarnaev was wounded but survived.
Tazhayako and Kadyrbayev appear in a photograph with Dzhokhar Tsarnaev taken in New York's Times Square during an earlier visit. They were taken into custody last month on charges that they had violated the terms of their student visas and had already been questioned by investigators on April 19, hours before Tsarnaev's arrest, Kadyrbayev attorney Robert Stahl told CNN.
Federal law enforcement sources said at the time that the Kazakh students were being detained "in an abundance of caution" because authorities wanted detailed information on the Tsarnaevs' movements in the weeks and days before the attack.