The gunman burst into the Resorts World Manila casino, fired shots and set gaming tables alight in the Philippine capital early on Friday.
He later killed himself but police said this morning they were searching for a "person of interest" in connection with the attack.
Shots and loud bangs – believed to be explosions – rang out shortly after midnight (4pm Thursday GMT) in the buildings close to Terminal Three of Ninoy Aquino International Airport and an air force base.
ISIS has seized control of the city of Marawi in another part of the country and people feared the casino shooting was a terrorist attack.
At least 36 bodies have been found in the resort after the attack.
Presidential spokesman Ernesto Abella said: "All indications point to a criminal act by an apparently emotionally disturbed individual.
"Although the perpetrator gave warning shots, there apparently was not indication that he wanted to do harm or shoot anyone."
Police said the incident appeared to be an armed robbery – with the gunman trying to steal gambling chips.
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The gunman is described as white, tall and English-speaking.
Police said he had committed suicide by setting himself alight.
Oscar Albayalde, chief of the Manila police office, said: "Our assessment is he burned himself. He committed suicide."
Smoke was seen billowing from the upper story of the resort complex's hotel wing hours after the initial assault as police worked to clear the resort room by room.
Most of the dead are thought to have suffocated from the smoke.
Footage posted on social media showed screaming hotel guests fleeing for their lives as gunshots are heard.
Fire, police and SWAT teams surrounded the resort.
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At least another 50 people were hurt in the attack – some seriously – as they rushed to escape.
All guests were evacuated.
ISIS claimed responsibility for the attack, according to terror-monitoring website SITE.
But local police say the incident was actually a robbery carried out by a lone gunman.
One employee reportedly heard five shots fired on the casino’s third floor before the building was evacuated.
National police chief Ronald dela Rosa said the gunman had pointed his rifle away from people in the casino.
Mr Dela Rosa said: "We cannot attribute this to terrorism.
"We are looking into a robbery angle because he did not hurt any people and went straight to the casino chips storage room.
"He parked at the second floor and barged into the casino, shooting large TV screens and poured gasoline on a table setting it on fire."
Resorts World Manila sits opposite the Ninoy Aquino International Airport Terminal 3 – which is one of the busiest airport terminals in the country.
It is famous for its casino, shopping mall, cinema and theatre, and is described as "the first and largest integrated resort in the Philippines".
The resort said earlier on Twitter: "Resorts World Manila is currently on lockdown following reports of gunfire from unidentified men.
"The company is working closely with the Philippine National Police to ensure that all guests and employees are safe.
"We ask for your prayers during these difficult times."
US President Donald Trump has already reacted to the attack with a statement during his press conference in which he announced the US are pulling out of the Paris Climate Agreement.
He started the anouncement with: "I would like to begin by addressing the terorrist attack in Manila.
"We are closely monitoring the situation and I will continue to give updates if anything happens during this period of time.
"It is really sad as to what is going on through the world with terror.
"Our thoughts and our prayers are with all of this affected."
This comes after a terror expert warned Daily Star Online that ISIS supporters were planning to hijack the month of Ramadan to carry out attacks.
It also comes as tensions between the Philippines and ISIS have been rising in recent months with jihadi fighters ransacking the country with attacks.
Philippines president Rodrigo Duterte recently vowed to wipe out the group after declaring martial law in Marawi City.
And just yesterday Duterte declared martial law in the Philippino island of Mindanao following further clashes between government forces and a group inspired by ISIS.
The martial law allows the suspension of ordinary law, meaning the country can be more flexible with its military approach.
Earlier this week Duterte warned rebel groups fighting in his country that he will “throw them in the slammer” if they help ISIS.