It is now the deadliest shooting in modern U.S. history.

Sheriff Joe Lombardo says both numbers are estimates, as the investigation is still underway.

Dozens of police vehicles descended on the area after authorities received reports of an active shooter near the Route 91 Harvest Festival.

Some officers took cover behind their vehicles, while others carrying assault rifles ran into the Mandalay Bay Hotel and Casino.

He says two officers are in hospital -- one of them in critical condition and the other with minor injuries.

Authorities say the shooter, a Las Vegas resident identified as 64-year-old Stephen Paddock, has been killed.

They believe it was a so-called ``lone wolf'' attack.

UPDATE: 

Investigators have located 62-year-old Marilou Danley, who was wanted as a person of interest in connection with the attack.