Investigators prepared Sunday for a chance to interview the surviving suspect in the Boston Marathon bombings as they turned to finding a motive for the bombing. Dzhokhar Tsarnaev, 19, was in serious condition and unable to speak because of a bullet injury to his throat before his Friday capture. Investigators have zeroed in on a trip made by his brother, Tamerlan, to Moscow, in January 2012. It is still unclear whether Tamerlan could have had contact with militant Islamist groups in Southern Russia while there. Tamerlan was killed in a gun battle with police early Friday. Authorities have yet to charge the surviving Chechen brother, who is represented by a public defender. Pressure cooker bombs packed with ball bearings and nails killed three people and injured 176 at the Boston Marathon Monday.