It's said to be more than 50 people killed and 200 injured when a lone gunman opened fire from a perch high up in the Mandalay Bay Resort and Casino in Las Vegas Sunday night, police said, making it the deadliest shooting in modern U.S. history.

The "nonstop gunfire," according to one witness, sent bystanders outside the resort on the Vegas strip ducking for cover and scrambling for their lives. Tourists hid in their hotel rooms and flights headed into the McCarran International Airport were held elsewhere.

Bystanders sprang into action, caring for the wounded and at least one described someone dying in their arms.

Police said a Las Vegas police officer who was off-duty attending the concert is among the dead. 64 year old Stephen Paddock has been identified as the shooter, acting alone as a lone wolf shooter sources say.