The Las Vegas gunman meticulously planned how to carry out the worst mass shooting in modern US history, researching Swat tactics, renting other hotel rooms overlooking outdoor concerts and investigating potential targets in at least four cities, authorities said on Friday.

But months after Stephen Paddock killed 58 people and wounded more than 800 others with a barrage of bullets from the Mandalay Bay casino-hotel, investigators still have not answered the key question: Why did he do it?

On Friday, Clark County Sheriff Joe Lombardo released a preliminary report on the October 1 attack and said he did not expect criminal charges to be filed against Paddock's girlfriend, Marilou Danley, who had been previously called a person of interest in the case.