The suspected San Bernardino attackers left behind 12 pipe bombs and more than 2,000 rounds of ammunition at their home, police said Thursday.
Wednesday's shooting that killed 14 people and wounded 21 at social services facility The Inland Regional Center has been called the nation's deadliest gun massacre since Sandy Hook.
The guns used by the couple suspected of the massacre were purchased legally in the United States, according to federal officials. The attackers had purchased their two handguns, but their two assault-style rifles were purchased by someone who is now under investigation, San Bernardino police chief Jarrod Burguan told reporters on Thursday. The shooters wore black tactical gear, but did not have ballistic vests, he added.
NBC News learned Thursday that the attackers left behind a remote-controlled explosive device at the scene of the shooting as they left in their black SUV with the controller to the bomb in their hands.
Multiple federal and regional sources familiar with the investigation told NBC News that the controller was similar to the model car controller used in the Boston Marathon bombing. This would indicate that the attackers appear to have calculated a heavy law enforcement and public safety response and left behind a device that could have caused even greater causalities. Burguan said the remote appeared not to have worked for them.
California governor Jerry Brown has since called for increased caution.
At least 75-80 people were in the room when the shooting occurred, Burguan said, adding that the attack took place at the Center's department for public health. The majority of the victims were county employees, he noted.
While the couple appeared to have planned the attack, the motive has not been determined, investigators said. At least 10 of the 21 wounded were in critical condition early Thursday, NBC reported. Authorities will start releasing names of victims "fairly soon," Burguan said.