Law enforcement are now acknowledging that the San Bernardino attack was an act of terrorism, and ISIS may have been involved.
The new puzzle authorities have to solve? There isn't any evidence that the terror group actually planned the shooting. Experts say this could be due to ISIS's new strategy when it comes to its supporters.
Officials do not believe ISIS planned the San Bernardino mass shooting, but that the group influenced the shooters online.
"It's saying, 'look you don't have to come to Syria and join the Islamic State, you can help the cause by simply staging San Bernardino, Paris, other types of attacks'," Nader Hashemi, of the Center for Middle Eastern Studies at Denver University said.
The female shooter allegedly pledged support to ISIS online before opening fire on a holiday party with her husband, killing 14.
"This is a process of self-radicalization. People reading stuff online, buying into it and then staging these attacks," Hashemi said.
"It's a real concern how such people could be radicalized from afar," Christopher Hill, Dean of the School for International Studies at DU, added.
Experts on International and Middle Eastern Studies at DU say this new way of rallying support for the ISIS cause, from across the world asking supporters to carry out their own attacks, makes the threat of terrorism even more difficult to deal with.
"You can stop people from coming into the country, you can screen them but you can't stop the internet. You can't shut down social media," Hashemi said.
"So it's really going to force our law enforcement agencies to think long and hard about how to try to anticipate this issue," Hill added.
Another problem with this new way of asking for support by ISIS, according to Hashemi, is that even if the group was uprooted and destroyed today, the ideas they've put out there would still pose a possible threat from supporters worldwide.