A guard was shot during the melee but survived, and more than 70 others suffered mostly minor injuries in a stampede to get away from the gunman, who was wielding an assault rifle.
The attack sent hundreds fleeing into the night and produced an immediate claim of terrorism from an Islamic State-affiliated operative, according to US terror monitors.
But within three hours of the violence at the complex near Manila's airport, police stressed that they uncovered no ties to terror and suggested the motive could have been robbery.
“He would have shot all the people gambling there” if it had been terrorism, said the national police chief, Gen. Ronald dela Rosa. “But he did not hurt anyone.”
Added dela Rosa: “Do not panic. This is not a cause for alarm. We are just alert. ... We cannot attribute this to terrorism without concrete evidence.”
The security guard shot was the waist by the gunman and about 75 others suffered mostly minor injuries such as bruising as they stampeded to get out, said police officer Jeffrey Francisco.
Police said there was no indication any hostages had been taken by the lone gunman, who was still at large.
The violence unfolded as Muslim militants aligned with the Islamic State group fended off government forces for a 10th day in the southern city of Marawi.
That unrest had sparked fears that the militants might attack elsewhere to divert the focus of thousands of troops trying to quell the siege.
In Friday's attack, the gunman stole gambling chips, shot TV screens and set gambling tables ablaze by pouring gasoline on them at Resorts World Manila, dela Rosa said.
But he said the assailant did not fire at people he encountered. The national police chief described him as “white, with a mustache” and about 6 feet tall.
It was not clear how the gunman smuggled gasoline and an assault rifle into the crowded casino or what prompted dela Rosa to cast doubt on terrorism so quickly.
Dela Rosa said CCTV footage showed the gunman ignoring a security guard who tried to question him at the entrance to the complex. He did not hurt the guard but went straight to the gambling area, dela Rosa said.
Ronald Romualdo, a maintenance worker at Resorts World, said he and his colleagues heard gunshots and saw people smashing the windows on the second floor and third floor to escape.
“We took out a ladder to save them. We were able to save many of them,” he said. “But one woman I was trying to save fell from the second floor. ... I could not carry her. She was not moving and was probably dead.
“I got her bag so her family will know what happened to her,” he said. “I don't know if she is still there.”
About 90 minutes after the attack began, Resorts World Manila said on its Facebook page that it was “on lockdown following reports of gunfire from unidentified men,” adding: “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,” the company said.
Philippine police cordoned off the area near the Ninoy Aquino International Airport.
“I heard many, many gunshots,” Julio Silva, a casino player who managed to dash out of the mall complex amid the gunfire, told DZMM radio network.
Silva said he saw a SWAT member who was shouting: “I was hit, I was hit.”
As news of the attack spread, US President Donald Trump offered the thoughts and prayers of the American people to the Philippines.
“It is really very sad as to what's going on throughout the world with terror,” he said from the White House Rose Garden. Trump said he was “closely monitoring the situation” and would continue to provide updates.
The SITE Intelligence Group, a US terrorism monitor, said an Islamic State-linked Filipino operative who provides daily updates on the ongoing clashes in Marawi claimed “lone wolf soldiers” of the Islamic State group were responsible for the attack.
An English message by the operative was distributed across several pro-IS Telegram chat groups, SITE said. According to SITE, he wrote: “The lone wolf soldiers of Khilafah attack the heart of Kufar the city of Manila in Resort World.” (AP)