A preliminary examination of the cellphones and computers used by the Tsarnaev brothers has found no indication of an accomplice in the Boston Marathon bombing, according to a U.S. counter-terrorism source briefed on the FBI investigation. The source stressed that the investigation is ongoing, but bureau officials at this point appear increasingly confident that “nobody else was involved,” said the source. Dzhokhar Tsarnaev has told investigators he and his brother acted alone, learned to build the pressure-cooker bombs over the Internet and were motivated by a desire to defend Islam because of "the U.S. wars in Iraq and Afghanistan," said...