The radio news said that the coast guard had received a telephone call from a woman who threatened to blow up the bridge. This is slightly at odds from the Portland press Herald that was reporting the threat was over marine channel 16 - the distress channel. As the radio report was just given (4:04 pm) I think its probably more up to date. I suppose its possible that there could have been 2 threats.
No information on the nationality of the woman, but if a phone call was made, I'd imagine that it could be traced fairly easily.