(JTA) — The FBI is treating the inquiry into the mass shooting in San Bernardino, California, as a counterterrorism investigation.

The decision follows the discovery that the couple believed to have opened fire Wednesday at a social services center’s holiday party, killing 14, had stockpiled thousands of rounds of ammunition and pipe bombs in their home.

The couple, Syed Rizwan Farook, 28, and Tashfeen Malik, 27, who had a 6-month-old daughter, also had traveled to the Middle East and been in contact with more than one person being investigated by the United States for terrorism, The New York Times reported Thursday, citing unnamed law enforcement officials.

The unnamed officials told the Times that the FBI had uncovered evidence that Farook was in contact over several years with Islamic extremists domestically and abroad.

Farook and Malik were killed in a shootout with police hours after the attack, which also injured 21.

Farook, a U.S. native, had worked for the San Bernardino County health department for five years and attended the holiday party for county staff at the Inland Regional Center, which provides services for developmentally disabled residents. He reportedly left in anger before returning with his wife. The two then allegedly shot up the center, focusing on the party. Malik held a Pakistani passport.

“We do not yet know the motive,” David Bowdich, the assistant director of the FBI office in Los Angeles, told reporters on Thursday. “We cannot rule anything out at this point.”

The attack also could have involved workplace grievances, according to reports. But officials told the Times that the shooting was clearly premeditated.

“There appears to be a degree of planning that went into this,” San Bernardino Police Chief Jarrod Burguan told reporters. “Nobody just gets upset at a party, goes home and puts together that kind of elaborate scheme or plan.”

The four guns used in the attack were all purchased legally. The couple wore black tactical gear and vests that held equipment during the attack.

President Barack Obama, who announced Thursday that the FBI had taken over the investigation, was not definite about the motive behind the attack.

“At this stage, we do not yet know why this terrible event occurred,” Obama said during remarks at the White House. “It is possible this was terrorist-related, but we don’t know. It is also possible this was workplace related.”

San Berardino was the deadliest mass shooting in the United States since the Sandy Hook massacre in 2012 claimed the lives of 26 at a Newtown, Connecticut, elementary school, including 20 first-graders.

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