BOSTON (AP) — The FBI agent in charge in Boston says whoever carried out the bombing at the marathon "is someone's friend, neighbor, co-worker or relative."

And that's why authorities are continuing to appeal to the public to report anything they may have seen or heard, even before the attack.

Officials say the theory is that someone knows who bombed the race, killing three people and wounding more than 170.

The bomber may have been seen amid revelers, carrying an unusually heavy nylon bag, weighed down with shrapnel-packed explosives.