The US Federal Bureau of Investigation (FBI) says there is no indication that the gunman who killed four American Marines in Chattanooga, Tennessee, was inspired by the ISIL terror group.

Ed Reinhold, the FBI special agent in charge, told reporters Friday that investigators believe 24-year-old Mohammad Youssuf Abdulazeez acted alone when he attacked two military facilities on Thursday.

However, Reinhold said the shootings were being treated as an "act of terrorism" until evidence shows otherwise.

"We are saying that is a possibility that we will explore, but we have no indication that he was inspired or directed by anyone other than himself," the FBI agent said.

Abdulazeez, a Kuwaiti born Jordanian who became a naturalized US citizen, took a seven-month trip to Jordan last year, a federal law enforcement official said Friday.

Abdulazeez opened fire Thursday at two locations six miles apart, killing four Marines at the US Naval Reserve Center and injuring three people, before he was killed in a firefight with police.

Fifth sailor dies

The US navy said on Saturday that a sailor who was shot at the military recruiting center in Chattanooga has succumbed to his injuries. The sailor’s death brought the toll from the shooting to five.

There have been 281 mass killings in the US over the past decade, including 213 fatal shootings of four or more people since 2006, according to data gathered by USA Today.

Although the United States has witnessed countless mass shootings since the September 11, 2001 attacks, mostly shootings carried out by Muslim Americans get classified as acts of terrorism, even as recent studies show that white supremacist and radical anti-government groups pose the greatest domestic terror threat in the country.

US 'double standard'        

“It’s sort of an American double standard, and that partly has to do with religion, but it’s also partly what’s in our consciousness,” says Donald Haider-Markel, professor and chair of the political science department at the University of Kansas. “Generally speaking, most people aren’t paying attention to most acts of terrorism in the country, most of which at this time are by right wing groups.”

American media outlets have been reluctant to classify last month’s Charleston, South Carolina, church massacre as terrorism, despite the fact that it fits the definition of a terrorist act, defined as extreme violence intended to murder civilians and to create fear, based on political and ideological beliefs.

Dylann Roof, 21, the white gunman who shot nine black people dead at the Emanuel African Methodist Episcopal Church in Charleston on June 17, has been described by mainstream media outlets and authorities as mentally ill and filled with the hatred of blacks, not a terrorist.