The FBI is investigating the mass shooting in San Bernardino, California, “as an act of terrorism.”

David Bowdich, the assistant FBI director at the bureau’s Los Angeles office, made the announcement at a news conference on Friday, saying the bureau had taken over the investigation’s lead from local authorities.

The revelation came two days after the husband-and-wife team of Syed Rizwan Farook and Tashfeen Malik killed 14 people and injured 21 others when they opened fire during a holiday party at the Inland Regional Center. They were later killed in a shootout with police.

Bowdich told reporters that investigators found evidence that indicated “extensive planning,” as well as mentioning the couple’s extensive stockpile of ammunition and explosive devices.

Speaking at a later news conference in Washington, FBI Director James Comey told reporters there was evidence that suggested the Muslim suspects became radicalized, but no evidence had surfaced that suggested that the shooters were part of a terrorist group. However, Comey did say there were indications that the couple might have been inspired by foreign terrorist organizations.

“There’s a lot of evidence in this case that doesn’t quite make sense,” Comey told reporters, adding that investigators were still wading through a large amount of electronic evidence.

Neither Bowdich nor Comey publicly confirmed multiple published reports that said Malik pledged allegiance to the leader of the Islamic State through a post on social media on the morning of the attack.

CNN, FoxNews, The New York Times and The Washington Post were among the media outlets reporting that sources familiar with the investigation told them about the Facebook post.

Bowdich told reporters at the news conference that Farook and Malik “attempted to destroy their digital fingerprints,” and investigators discovered two “crushed” cell phones in trash cans near the crime scene. He said authorities were working to extract data from those cell phones.

“We do hope that the digital fingerprints that were left by these two individuals will take us towards their motivation,” Bowdich told reporters. “That evidence is incredibly important.”

Comey told reporters neither Farook nor Malik was on the FBI’s radar at the time of the shooting.

It was widely reported that Farook worked at the San Bernardino County Department of Public Health, which was holding a holiday party at the Inland Regional Center at the time of the attack. Police said Farook left the party angrily, then returned with his wife and opened fire.

Farook was born to Pakistani parents in Illinois and met Malik, a Pakistan native who was living in Saudi Arabia, on an online dating site, the New York Times has reported. The couple had a six-month-old daughter who they left with her grandmother on the morning of the attack.

Police have said they found at least 4,500 additional rounds of ammunition and a dozen “pipe-bomb-type devices” at the Redlands, California, residence of the suspects.

The FBI released the crime scene and the couple’s landlord somewhat bizarrely opened it up to the media on Friday morning, leading to several news organizations to take cameras through the residence and broadcast live images of reporters examining the suspects’ personal belongings.