After a regional manhunt, the two perpetrators were killed by police after gunfire was exchanged with them while in their vehicle, approximately four hours later. Police emphasized that no motive had yet been discovered. The FBI took over the investigation as a counter-terrorism investigation due to the equipment used, recent travel to the Middle East and one of the perpetrators potentially having contacts with Islamist extremist views. Additionally Malik was reported to have pledged allegiance to the Islamic State of Iraq and the Levant and its leader Abu Bakr al-Baghdadi.
The attack was the second-deadliest mass shooting in California's history, after the 1984 San Ysidro McDonald's massacre, and the deadliest in the U.S. since the 2012 Sandy Hook Elementary School shooting.
Weapons and equipment usedThe shooters were wearing black tactical gear, but not bulletproof vests, and armed with rifles and handguns. Investigators reported that the attackers used two .223-caliber semi-automatic rifles, two 9mm semi-automatic handguns, and an explosive device in the attack. All four of the guns were purchased legally in the U.S. four years before the attack, and two of them were purchased by a person now under investigation.
The rifles used were variants of the AR-15: one was manufactured by DPMS Panther Arms, the other was a Smith & Wesson M&P15. One of the handguns was manufactured by Llama and the other by Smith & Wesson. The ATF stated that the firearms were all purchased in California. The rifles were subsequently and illegally altered to make them more powerful: there was an attempted modification to enable the Smith & Wesson rifle to fire in fully automatic mode, and the DPMS weapon was modified to use a high-capacity magazine. The couple had 1,400 rounds for the rifles and 200 for the handguns with them at the time of the shootout.
In addition to the two firearms, the attackers left an "explosive device" (three explosive devices connected to one another) at the Inland Regional Center; this device was later detonated by a bomb squad.