Phillipos is charged with lying to federal agents investigating the bombing. He faces up to eight years in prison if convicted, along with a $250,000 fine.
A friend described Phillipos as a "good kid."
"He went to school, never got in trouble, took care of his mom," James Turney told CNN affiliate WBZ. "He was not really outgoing, stayed in the house a lot, did homework, got good grades."
Phillipos plays basketball and doesn't have "any anti-American thing about him," Turney said.
"It just doesn't make sense. Robel doesn't have anything to do with this, or what happened, so I don't see why he's being arrested," he told WBZ.