U.S. investigators believe that they have identified a suspect for Monday's Boston Marathon bombings, which killed three and injured over 170, U.S. media reported on Wednesday.

"Substantial progress" has been made in the investigation into Monday's bombings near the Boston Marathon finish line, CNN quoted official sources as saying.

The investigators also said that video provided by a local store and a TV station had helped the investigation.

Law enforcement officials planned to hold a briefing on the progress later Wednesday afternoon, according to media reports.

Twin explosions occurred on Monday afternoon near the finish line of the Boston Marathon, causing heavy casualties. The authorities have called the bombings an "act of terror," but the motives and perpetrators remain unknown.