At least 14 people were killed after gunmen opened fire at a social service agency in the Southern California city of San Bernardino on Wednesday.
An intense manhunt for up to three suspects, believed to be wearing military-style gear, was underway and the San Bernardino police reported one “suspect down” in an exchange of gunfire.
It was unclear if the suspect was one of the gunmen, but police earlier said the attackers may have fled in a dark SUV.
Police gave no details on a motive for the nation's latest mass shooting.
"They came prepared to do what they did, as if they were on a mission," San Bernardino Police Chief Jarrod Burguan said, noting the shooters carried long guns, not handguns.