At least 50 people were killed and more than 400 people were hospitalized after a lone wolf opened fire at the Las Vegas mass shooting Sunday — but the violent act was not immediately deemed one of terrorism. Twitter users wondered why Stephen Paddock, the accused assailant, wasn't labeled a terrorist. Some claimed the color of his skin and religion had something to do with it.

An act of violence is deemed an act of terrorism depending on the motivation.

“Although acts of war, acts of crime and acts of terror can look very much alike on the surface, they have very different motives, very different reasons for being, and I think that's why people are confused,” Robin Lakoff, a linguist at the University of California, Berkeley, told NPR in 2015. “They look alike on the surface; they're different underneath.”

Still, Lakoff argued “terrorism itself, whatever that is” has a tricky definition. According to Merriam-Webster, it’s defined as “the systematic use of terror especially as a means of coercion.”

Back in 2011, the U.S. declared war against “terrorism” after Sept. 11, not many people knew what the word meant—not even the U.S. government. “No one definition of terrorism has gained universal acceptance,” the State Department said in a report on world terrorism in 2000, as per ABC News.

The FBI has another definition. It is “the unlawful use of force and violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives,” the Code of Federal Regulations reads.

Paddock, 64, did not have a criminal past and his motive wasn’t immediately known.

“All of the checks that we have been able to do other than a routine traffic violation here in Nevada and nationwide working with our local FBI partners have been able to find no derogatory history on that individual,” Undersheriff Kevin McMahill of the Las Vegas Metropolitan Police Department said. “It’s one of those really sad, tragic things that a man that’s 64 years old that really had no other reason that we can find at least in his history here to go out and wound that many people.”

Paddock reportedly killed himself after conducting the deadliest mass shooting in U.S. history. Previously, 49 people were killed at Pulse nightclub in Orlando last year.

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