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Since lone-wolf terrorists are difficult to spot and hard to stop, the U.S. government should be taking more aggressive measures to track potential threats before they erupt in violence like Thursday’s mass shooting in Tennessee, according to a counter-terrorism expert who appeared on The Laura Ingraham Show Friday.

Steve Emerson, executive director of the Investigative Project on Terrorism, said it is not unusual for attacks to be carried out by radicalized Muslims — as in the case of Tennessee shooter Mohammad Youssef Abdulazeez — described by acquaintances as well-assimilated.

“They said it about the 9/11 terrorists,” he said.

“That’s exactly the path,” Emerson said. “It follows a trajectory. … It’s part and parcel of that narrative that the Islamist groups put out.”

Abdulazeez shot four Marines dead and wounded three other people at a pair of recruiting stations in Chattanooga, Tennessee. (Update: A fifth service member died early Saturday morning, according to the Navy.) Armed with an AK-47, Abdulazeez held police at bay before he died Thursday. Friends and acquaintances have expressed surprise, telling reporters that the 24-year-old college graduate was well-liked and friendly. They described the Kuwaiti-born American citizen as devout but not overly religious.

Emerson recalled similar descriptions of Mohamed Atta, one of the ringleaders in the hijacking of three airplanes on Sept. 11, 2001. “They said it about the 9/11 terrorists,” he said.

Abdulazeez apparently was not on any terrorism watch list.

Abdulazeez apparently was not on any terrorism watch list. Emerson called for unshackling the FBI.

“Should the FBI be allowed to create a preliminary intelligence database?” he asked. “That jumping-off point (to violence) is so … hard to predict.”

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Emerson also disputed the contention by the Southern Poverty Law Center and the left-leaning New American Foundation that white, right-wing radicals pose a bigger terrorism threat to the U.S. homeland than Islamic jihadis. He called it “totally intellectually untrue.”

Emerson said that deaths caused by right-wing extremists are higher only because the FBI has been less successful at stopping them.

Factoring in all of the plots that counter-terrorism officials have broken up, the number of potential victims of Islamic terrorists far exceeds the number of potential victims of right-wing radicals, Emerson said.

This article has been updated to reflect that a fifth service member has now died as a result of Thursday’s shooting.