Washington: Investigators believe the couple who gunned down 14 people last week in San Bernardino, California, planned their attack.

Shooters Syed Rizwan Farook and Tashfeen Malik practiced at gun ranges in the Los Angeles area, said David Bowdich, assistant director of the FBI's Los Angeles field office. And on at least one occasion, he said, their target practice occurred within days of the shooting.

They "were radicalized and have been for quite some time," Bowdich said.

"The question for us," he told reporters Monday, "is how, and by whom, and where."

And as authorities comb through evidence five days after a massacre that President Barack Obama and law enforcement officials call an act of terrorism, there's another key question looming over the investigation: Could anything have been done to foil the plot?