WASHINGTON - US President Barack Obama vowed on Saturday that federal investigators would find out what motivated a married couple in California to shoot and kill 14 people, and he asked Americans to stand united after the attacks."We are strong. And we are resilient. And we will not be terrorized," Obama said in his weekly address.Tashfeen Malik and Syed Rizwan Farook were killed in a shootout with police after the Wednesday attack during a holiday party at a social services agency in San Bernardino, California.The Federal Bureau of Investigation is investigating the massacre as an "act of terrorism" after Malik was believed to have pledged allegiance to a leader of the militant group Islamic State."It is entirely possible that these two attackers were radicalized to commit this act of terror," Obama said. "And if so, it would underscore a threat we've been focused on for years - the danger of people succumbing to violent extremist ideologies."