One was seen placing a dark-colored backpack outside the Forum restaurant, the site of the second bombing, just minutes before the explosion, said Richard DesLauriers, the special agent in charge of the F.B.I.'s Boston field office.
“Today we are enlisting the public’s help to identify the two suspects,” Mr. DesLauriers said at a news conference on Thursday evening in Boston.
In the video, both men are carrying backpacks, and wearing baseball caps, one a dark cap and one a white cap turned backward. They are walking along Boylston Street.
After a concerted effort, he said, investigators determined that a second suspect had been involved.
Both men appeared to be wearing dark-colored zippered-front jackets. The first, whom Mr. DesLauriers called Suspect 1, wore a dark-colored baseball cap with a white emblem on it and markings on the front, a white T-shirt and tan pants. Visible around the edges of his cap was short dark hair. The man he identified as Suspect 2 wore a white baseball cap backward, with dark-colored pants. He had slightly longer curly hair. The men appeared to be wearing hooded sweatshirts beneath the jackets.
Mr. DesLauriers did not characterize the appearance of the men or offer an opinion as to their possible ethnicity or national origin.
Mr. DesLauriers urged anyone who has seen the men, or who knows who they are, to call 1-800-CALL-FBI (1-800-225-5324).