People ran screaming out of Resorts World Manila, which is across a road from the Philippines' international airport, after the man fired what police chief Ronald dela Rosa said was an M4 assault rifle and set fire to a gambling table about midnight.
The gunman did not shoot anyone but disappeared into the chaos of smoke and running people, leading to a manhunt throughout the casino, hotel and shopping complex that ended just after dawn.
Before the gunmen had been killed and police had confirmed any motive, the Islamic State group claimed responsibility and US President Donald Trump also branded it a "terrorist attack" .
However the incident was most likely a robbery gone wrong rather than a terrorist attack, Dela Rosa and Manila police chief Oscar Albayalde told reporters.
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Dela Rosa said the man, who appeared to be acting alone, walked into one of the gambling rooms and fired the rifle at a large television screen, then poured gasoline onto a gambling table and set it alight.
The man left the room and went upstairs to the hotel section, according to Dela Rosa.
Albayalde said the gunman had tried to steal 113 million pesos (USD 2.3 million) worth of gambling chips, although he left the backpack on the ground near the stock room.
"There is no indication that this is a terror attack," Albayalde told reporters outside the casino.
"It is a simple robbery and most likely it was done by a demented person."
Dela Rosa was less equivocal, saying that it appeared not to be a terrorist attack but that an investigation would determine the gunman's motive.
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