Stephen Paddock opened fire on a huge crowd of music fans from the 32nd floor of the building, killing 58 people and injuring almost 500.
Police found Paddock dead after bursting into his hotel room following the massacre, with photos showing a notebook on a nightstand.
Now officers have revealed what was written in the notebook, giving a sick insight into the moments leading up to Paddock spraying the crowd of 22,000 people.
Officer David Newton said: “I could see on it he had written the distance, the elevation he was on, the drop of what his bullet was going to be for the crowd.
“So he had had that written down and figured out so he would know where to shoot to hit his targets from there.”
The notebook meticulously laid out how to kill the most people from his vantage point high above the helpless crowd.
Officer Newton also detailed the horrifying moment police finally breached the attacker’s hotel room, in scenes he said were like something “out of a movie”.
He said: “Very eerie. Yeah, the dust from the explosive breach and then you have the flashing lights.”
Investigators are still desperately attempting to piece together what exactly happened on the night of the deadliest shooting in US history.
Shortly before 10.05pm local time Paddock used a hammer to break two windows in his luxury Mandalay Bay resort and casino suite.
As the final performance at the Route 91 Harvest country music festival wound down, Paddock began shooting thousands of bullets into the packed crowd.
For 10 minutes the crowd attempted take shelter, with some simply lying on the ground, as Paddock picked off victims.
At 10.15pm or so the shooting stopped. Two minutes later a security guard arrived at his door and was injured when Paddock fired some 200 bullets through it.
At this point armed police arrived on the floor and began searching every room. At 10.55pm they reached Paddock’s room, forcing their way in around half an hour later.
They found Paddock dead on the floor, with officers quickly spotting the note on a table near his body.
Investigators are still clueless as to Paddock’s motive – but are not ruling out he may have had help in carrying out the slaughter.
Earlier this week Sheriff Joseph Lombardo said: “Do you think this was all accomplished on his own. You’ve got to make the assumption he had to have some help at some point.”