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David Bowdich, assistant director of the FBI's Los Angeles bureau, told reporters at a press conference that based on the evidence, the FBI is "now investigating these horrific acts as an act of terrorism."
"We have uncovered evidence that has led us to learn of extensive planning," Bowdich said. "Obviously we've uncovered evidence of explosives, multiple armaments ... high powered weapons, the explosive devices."
The shooters also allegedly tried to cover their digital tracks.
"We have also uncovered evidence that these subjects have attempted to destroy their digital fingerprints," Bowdich said. "We found two cellphones in a nearby trash can. Those cellphones were actually crushed. We have retained those cellphones and we do continue to exploit the data from those cellphones."
The shootings killed 14 people and left at least 21 others injured at the Inland Regional Center on Wednesday. The attackers might have been inspired by the terrorist group ISIS (also known as ISIL and the Islamic State), according to officials.
Syed Rizwan Farook, a 28-year-old US citizen, and Tashfeen Malik, his 27-year-old wife, died in a shootout with police after they carried out the attack.
Police are investigating whether they might have been planning a second attack, Bowdich said.
Authorities believe that at least one of them had been in contact with suspected Islamic extremists, Bowdich said. Malik reportedly pledged allegiance to ISIS leader Abu Bakr al-Baghdadi in a post on a Facebook page that she operated under a false name, officials told news outlets on Friday.
Bowdich urged the public to not let the incident cause "mass hysteria."