Probe into how, when shooters became radicalized

Syed Rizwan Farook and Tashfeen Malik were radicalized, with the latter pledging allegiance to ISIS while the pair killed 14 people in San Bernardino, California. But when, and how, did they take this dark turn? Officials are trying to figure that out, as well as who might have guided or helped the married couple turn their extreme views into a horrific act of terrorism, as characterized by President Barack Obama and law enforcement officials. Sources told CNN that investigators believe Malik was radicalized at least two years ago, well before she came to the United States with Farook on a fiancee visa and before ISIS proclaimed its caliphate. Still, it's hard to say what precipitated her or Farook's shift, with one U.S. official noting, "It's not like a switch goes on and you're radicalized." "It's complicated," said another U.S. official. "They were looking to be radical and attach themselves to (a group)." Investigators are working to determine if the pair ever met or took orders from ISIS leaders, or if anyone outside the United States had a hand or even knew of their plans.