BOSTON — In a direct appeal for help from the public, the F.B.I. on Thursday released pictures and video of two young men who officials believe may be responsible for the explosions that killed three people and wounded more than 170 during the Boston Marathon.

Officials said they have images of one of the men putting a black backpack on the ground just minutes before two near-simultaneous blasts went off near the finish line of the marathon at 2:50 p.m. on Monday. One video, which officials said they did not release, shows the two men walking slowly away after a bomb exploded while the crowd fled.

At a news briefing here, Richard DesLauriers, the special agent in charge of the F.B.I.’s Boston field office, initiated the unprecedented crowd-sourcing manhunt by urging the public to look at the pictures and video on the F.B.I.’s Web site, fbi.gov. The two men appear to be in their 20s, but Mr. DesLauriers did not characterize their appearance or offer an opinion as to their possible ethnicity or national origin.