The FBI has said there are "no known additional threats" in Boston following the bombing of the marathon, according to BBC News.

At a news briefing in the city, Richard DesLauriers, the agent in charge of the investigation, said no bombs other than the two which exploded had been found.

Officials paid tribute to emergency workers and repeated their call for information from the public.

Three people were killed and more than 150 injured by the bombs, which went off close to the marathon finish line.