Attorney General Eric Holder said Tuesday the alleged terror plot disrupted in New York was "one of the most serious in the United States since Sept. 11, 2001" — but he gave no indication when more arrests might be made.

In a question-and-answer session with reporters, Holder said prosecutors will bring all those involved in the plot to justice.

So far, one suspect, Colorado shuttle bus driver Najibullah Zazi, has been charged with conspiring to detonate bombs in the United States. Zazi, who has denied any terrorist plotting, is being held without bond.