WATERTOWN - Police believe that Boston Marathon terror bombing suspect Dzhokhar Tsarnaev has been pinned down in a boat in the backyard of a Watertown home - and he is still alive, a source tells the Globe.

Police have seen the suspect sit up and are using “flash bang” stun grenades to disorient and distract him. Officers are acting with caution because they remain concerned that the suspect might be wearing a suicide bomb vest, the source said.

The source said police had seen the suspect moving from a State Police helicopter.