The couple accused in the mass shooting at a San Bernardino health center had been radicalized "for some time" and did target practice before the rampage left 14 dead and injured 21, the FBI said on Monday.
David Bowdich, the assistant director in charge of the FBI's LA field office, laid out more details during a news conference about the investigation into Wednesday's attack at the Inland Regional Center.
The wide-ranging probe involved interviews with more than 400 people and the collection of over 320 pieces of evidence, Bowdich said. The FBI said it has evidence that both shooters, identified as Syed Rizwan Farook, 28, and his wife, Tashfeen Malik, 27, participated in target practices at ranges across Los Angeles, including one days within of the shooting.
Bowdich also said both subjects had been radicalized and had been for some time, but it's unclear how long ago that occurred and by whom.
Agents are also trying to determine whether the attack was part of a broader plot, and if so, who might have financed it.
The news comes as federal authorities are scrutinizing the backgrounds of the assailants in the San Bernardino shootings to determine how they became radicalized and if they received outside funds to finance the deadliest terror strike on American soil since 9/11.
The Joint Terrorism Task Force has taken the lead in the investigation.
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President Barack Obama said Sunday night that the attack had been "an act of terrorism designed to kill innocent people."
Farook and Malik had been married two years and had a 6-month-old daughter, authorities said.
Intelligence sources said Malik posted a statement of allegiance to ISIS leader Abu Bakr al-Baghdadi just before the killing began, and an ISIS online radio broadcast said Farook and Malik were ISIS supporters.
"Two followers of Islamic State attacked several days ago a center in San Bernardino," media reports quoted the ISIS broadcast as saying. ISIS did not directly take responsibility for the attack, but praised Farook and Malik as "martyrs."
The couple was killed in a gunbattle with police about seven hours after the initial shooting at the Inland Regional Center, which provides treatment for people with developmental disabilities. Investigators found evidence Farook and Malik destroyed their mobile phones the day before the shootings.