Military investigators probing the deaths last November of about two dozen Iraqi civilians have evidence that points toward unprovoked murders by Marines, a senior defense official said Friday. The Marine Corps initially reported 15 deaths and said they were caused by a roadside bomb and an ensuing firefight with insurgents. A separate investigation is aimed at determining if Marines lied to cover up the events, which included the deaths of women and children. If confirmed as unjustified killings, the episode could be the most serious case of criminal misconduct by US troops during three years of combat in Iraq. Until now the most infamous occurrence was the Abu Ghraib prisoner abuse involving Army soldiers, which came to light in April 2004 and which President George W. Bush said Thursday he considered to be the worst US mistake of the entire war.