The U.S. faced another mass shooting Wednesday as authorities worked to piece together events in San Bernardino, California. As many as three gunmen may have been involved in the attack, authorities said.

Data from the Mass Shooting Tracker, a crowd-sourced initiative, showed that the majority of shootings in the U.S. are committed by individuals, or so-called lone gunmen. The Mass Shooting Tracker counts the number of people shot (not necessarily killed), and defines a mass shooting as those in which four or more people are shot in one setting or incident. The data analyzed spanned 2013 to 2015.

Of a total of 1,050 shootings recorded across the U.S., 760 were categorized as having an unknown gunman or woman. In the 290 cases where authorities were able to identify shooters, 251 were committed by lone shooters. There were 32 recorded cases involving two shooters. Five cases involved three shooters while only two cases involved four shooters.

"On The Media," an NPR program that focuses on journalism practices, issued a reporting guide in 2013 to help media consumers understand how to best read reports during mass shootings. The guide warns, “There’s almost never a second shooter.”

Recent high-profile shootings in the U.S. have involved lone gunmen. Robert Lewis Dear, 57, killed a police officer and two civilians in Colorado Springs, Colorado, when he opened fire at a Planned Parenthood healthcare facility Friday. Dylann Roof, 21, opened fire at the Emanuel African Methodist Episcopal Church in Charleston, South Carolina, killing nine people June 17 in what has been described as an attack fueled by racist ideology.

Over the history of mass shootings in the U.S., several well-known cases have involved multiple shooters who plotted the attack together, including the 1999 Columbine High School shooting in Colorado that left 12 students and one teacher dead. The shooting was committed by fellow students Dylan Klebold, 17, and Eric Harris, 18, who committed suicide at the scene.

The multiple shooters apparently involved in the San Bernardino case raised the question of whether the incident constituted domestic terrorism.