These lone wolf-style attacks are simple to execute and difficult to thwart, experts said. The attacks typically involve one or two people who support a certain ideology but have no direct links to an organization, which means the attackers are often flying under the radar of security officials. Simpson, 30, had been identified by the FBI as a jihadist terrorism suspect. But his fellow gunman Soofi, 34, was largely unknown to federal law enforcement, the New York Times reported.

“This is the way that ISIS is hoping to generate terrorist attacks against the U.S.,” said Max Abrahms, an expert on terrorism from Northeastern University in Boston. “Lone wolves have both the intent and the capability to continue mounting operations like this.”