One Down One To Go on Boston Killers

Dzhokhar Tsarnaev, 19, remains at large after he escaped a shoot-out in which another suspect, his brother, died.

Police said they had searched 60-70% of a locked-down area of a Boston suburb.

Three people died and more than 170 were hurt when two bombs exploded near the finish line of Monday's marathon.

On Friday afternoon, Col Timothy Alben of the Massachusetts State Police said officers in Watertown were searching "door to door, street to street" for the suspect, but there was as yet no word on his whereabouts.

"Things change, they change quickly," the officer said, adding: "We are working on several new leads that have just developed in the last few minutes."

Massachusetts Governor Deval Patrick said there were "continuing developments" in the investigation, and that an order to stay indoors remained in place across the whole of Boston and surrounding suburbs.

On Thursday evening, the FBI had released images of the two men they were hunting in relation to the bombing.

Police said one suspect - widely named in the US media as Tamerlan Tsarnaev - had been killed early on Friday, and they were looking for another suspect, later named as the dead man's younger brother Dzhokhar Tsarnaev.

Both are said to be of Chechen origin, and are reported to have moved to the United States about 10 years ago.