It’s being called the biggest massacre in U.S. history. A gunman opened fire late Sunday during a country music concert on the Las Vegas Strip, killing at least 50 and injuring more than 200, authorities said. Authorities said one on-duty officer is in critical condition and another was wounded in the shooting. Two off-duty police officers attending the concert were killed. The gunman, who fired down on the concert from a room on the 32nd floor of the Mandalay Bay Hotel and Casino, was killed by police. Law enforcement has announced the shooter’s identity as 64-year-old Stephen Paddock. Clark County Sheriff Joseph Lombardo said authorities believe this was a “lone wolf” attack but said they were looking for a roommate of the dead suspect as a person of interest. Authorities first received calls about an active shooting at about 10:08 p.m. Country music star Jason Aldean was performing at the Route 91 Harvest Festival when the gunfire erupted. Aldean was performing his last song of the night. Initially, those in attendance said they thought the sound was firecrackers. But as the shots continued, Aldean stopped singing and some concertgoers could be heard yelling to each other to get down. Authorities said law enforcement swarmed the hotel and killed the gunman in a room on the 32nd floor. The area is now on lockdown. All clubs and businesses on the strip are closed. University Medical Center spokeswoman Danita Cohen said the Las Vegas hospital is taking in people with gunshot wounds. She didn’t have any more immediate information.