(FOX NEWS) When Thomas Gunderson first heard the noise he thought a speaker had malfunctioned -- or maybe someone had shot off some fireworks.

But then he heard the screams -- and the 28-year-old from Newport Beach, California realized he was standing in the middle of a killing field.

Thomas Gunderson heard the first gunshots, he jumped into action -- helping people to safety outside the Mandalay Bay Resort and Casino.

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