UPDATE: The Suffolk County District Attorney's office reports the victim was 32 and from Boston. "Multiple witness statements suggest that the man who went over the bridge did so intentionally, and foul play is not suggested at this time." His body remains unrecovered.
The Boston Fire Department  reports a man jumped into the Charles River around 3:30 a.m. today:
Two men were walking from Cambridge to Boston across the bridge. About halfway across they hailed a cab. One went towards cab. The other jumped the fence into the Charles River.
Stanley Stacos reports  the man may have tried to swim out of the river but the current and cold may have proved too much for him.
Divers and firefighters from both Boston and Cambridge rushed to the bridge, which was shut as they searched in the water and from the bridge for the man's body, without success. State Police divers are now continuing the search for the body, in an area full of obstacles such as "rebar, metal and concrete," the department says.