This is troubling, but also a good reminder that early reports are notoriously flawed.

Las Vegas police offered new details Monday about the days and minutes leading up to last week’s deadly mass shooting, raising new questions about the police response and their investigation of the still unexplained massacre.

In a significant revision to the original timeline of the Oct. 1 massacre, authorities revealed that Stephen Paddock shot a security guard in the hallway outside his Mandalay Bay suite six minutes before he opened fire out the window on concertgoers at the Route 91 Harvest music festival, killing 58 people and injuring more than 500.

In their initial account, police said the security guard, Jesus Campos, was shot around 10:15 p.m. — about 10 minutes into the attack — when Paddock discharged a volley of gunfire through the door of his room after seeing Campos approach on a baby monitor the shooter had placed on a room service cart. Police had portrayed Campos as a hero, telling reporters he had interrupted and stopped the killing, and alerted law enforcement about the location of Paddock’s room.

But on Monday, police said Campos, who was unarmed, was shot and wounded at 9:59 p.m. as he investigated an apparently unrelated alarm for an open door on the floor — six minutes before Paddock began firing out his window at 10:05. Police now say they have no idea why Paddock, who had a large quantity of ammunition and other loaded weapons in his room, stopped his rampage 10 minutes later.

Speaking to reporters on Monday, Las Vegas sheriff Joseph Lombardo offered little explanation for the discrepancy in the accounts, although he implied that Campos, who was wounded and “extremely shaken up by what happened to him,” may have misremembered the details.