Police in Las Vegas are working to establish the motive behind the attack which left dozens of people dead.
Stephen Paddock opened fire on a country music festival from a balcony of the Mandalay Bay Hotel around 10pm local time (5am BST) on Sunday.
At least 59 people were killed, and more than 500 were injured by the gunfire.
Paddock later took his own life in a hotel room in which 23 guns were found, and so, as of the moment, the 64-year-old's motive is unknown.
Some reports say Islamic State has made a claim to the attack, but local investigators have not determined an official link to the organisation.
The group claims he converted to Islam several months before, but there's no evidence to substantiate this claim.
Psychological issues has also been reported as a potential cause, but is yet to be confirmed.
The shooting - the worst in US history - has been widely condemned by numerous political figures, like US President Donald Trump, UK Prime Minister Theresa May and Labour leader Jeremy Corbyn.