Emerging details suggest that Wednesday’s mass-shooting attack in San Bernardino may have been an extensive terror plot. Gunman Syed Rizwan Farook, who along with his wife Tashfeen Malik killed 14 people and injured at least 17 more after storming a social services center, had been in contact with “more than one international terrorism subject,” CNN reported on Thursday.

San Bernardino shooter Syed Rizwan Farook was in touch over the phone and via social media with more than one international terrorism subject who the FBI were already investigating, law enforcement officials said.

It appears that Farook was radicalized, which contributed to his motive, though other things — like workplace grievances — may have also played a role, other law enforcement sources said.

NBC added that Farook and Malik, both of whom were killed in a shootout with police, left an explosive device at the site of the attack. Authorities believe that the couple planned to detonate the bomb remotely after first responders arrived to the scene. They gave no reason as to why the device did not explode.

San Bernadino police chief Jarrod Burguan said that 12 pipe bombs were also found at the couple’s house. The explosives were similar to the ones used by the Tsarnaev brothers at the Boston Marathon in 2013. While no connection was established between the attacks, both the Tsarnaevs and Farook and Malik seemed to have built their bombs using instructions found on an online al-Qaeda magazine. Authorities are searching for suspects who may have helped the couple construct the devices and obtain the large amount of ammunition found in their house.

On Thursday afternoon, U.S. Secretary of Defense Ashton Carter expressed “enormous concern” about the danger of radicalization “of American citizens living in America,” saying that it was a reason to fight ISIS in Iraq and Syria.

[Photo: CBS Evening News / YouTube ]