The brothers intended to kill people and cause extensive material damage when they detonated "a powerful bomb" at the U.S. embassy in Oslo.
Police prosecutor Christian Hatlo said at a press conference that police are working on several hypotheses regarding motive.
"One of them is whether it involves an order from a government actor."
The three are all around 20 years old and live in Oslo; they are Norwegian citizens of Iraqi origin. None of them were previously known to Norwegian police.
No one was injured in the explosion, which occurred at 1:03 a.m. Sunday. It happened at the consular entrance and caused minor damage.
