Las Vegas gunman who killed 60 was angry at treatment by casinos, says FBI

The documents reveal the strongest indication of a motive for the deadliest mass shooting in modern American history.

Stephen Paddock had a number of assault weapons in his suite at the Mandalay Bay hotel in Las Vegas (Las Vegas Metropolitan Police Department via AP, File)

The high-roller gambler who opened fire on concertgoers on the Las Vegas Strip had lost tens of thousands of dollars while gambling weeks before the mass shooting, FBI documents have shown.

The files also show killer Stephen Paddock was upset at how the casinos had treated him.