On Wednesday, he said Kadyrbayev was accused of a "technical violation" of a student visa "for not regularly attending classes." 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.
Phillipos faces up to eight years in prison if convicted, along with a $250,000 fine; the charges against Kadyrbayev and Tazhayakov carry sentences of up to five years and $250,000 in fines.
Alan Dershowitz, a prominent defense attorney and Harvard law professor, called the obstruction charge "weak," suggesting it was meant to pressure the suspects into providing more information on Tsarnaev.
"If that's the best the feds have now, then they're just squeezing," Dershowitz told CNN. "It doesn't sound like they have very much new here."
One of the reasons Kadyrbayev drew investigators' attention was because of changes to his Facebook page, a source briefed on the Boston probe said. Kadyrbayev and Dzhokhar Tsarnaev changed their profile photos within 15 minutes of each other in the pre-dawn hours of April 19, while the Tsarnaevs were on the run, the source said.
Tsarnaev, who appears to have had access to a wireless device at that time, changed his to a black-and-white photo, while Kadyrbayev changed his photo to one of him wearing an Iron Man mask, the source said.