At approximately 11 a.m. on Wednesday, two shooters wearing black ski masks and bulletproof vests entered a end-of-the-year office party in the Inland Regional Center in San Bernardino, California and opened fire, killing 14 and wounding 21 others. The center, which provides services to people with developmental disabilities in Riverside and San Bernardino counties, employs almost 670 state health care workers.

Police shot and killed two of the suspects — one man and one woman — and captured a third suspect after a standoff. The suspect is currently being held in detention.  

“This is truly a tragedy in our country,” said FBI Los Angeles Director David Bowdich, who hinted that the shooting might have been terror-related.

“These are people that came prepared. They were dressed in a way to indicate they were prepared. They had long guns, not handguns,” said Police Chief Jarrod Burguan.

The mass shooting is the bloodiest since the Sandy Hook Elementary School shooting in Newtown, Connecticut three years ago, which claimed the lives of 20 schoolchildren and school teachers. It is the 355th mass shooting — defined by the FBI as a shooting in which four or more people, including the shooter, are injured or killed — that has taken place in the United States in 336 days,  one-shooting-per-day for 2015.

The shooting came only hours after a mass shooting in Savannah, Georgia that left one woman dead and three men wounded.

During a press conference Wednesday night, Loma Linda University Medical Center spokesperson Briana Pastorino told reporters that the hospital has admitted five adults wounded in connection with the shootings. Of those five, two are in critical condition, two are in fair condition and one is still being assessed.

According to an FBI study focusing on active shooter situations between 2000 and 2013, only two involved more than one shooter — one at a house party in South Jamaica, New York in August 2011 and the other on the streets of Tulsa, Oklahoma in April 2012. Mass shootings by lone gunmen are typically premeditated, but killings by multiple shooters often suggests more planning.

Many details of the shooting, such as the shooters’ motives or identities, are still under investigation.