According to the authorities report, Carlos, a former employee of the Philippines Department of Finance, recently lost his job and was a heavy gambler.
Police spokesman Oscar Albayalde said that Carlos had lost his job for withholding financial information from his employers and had accrued $80,000 in debt.
Police were initially skeptical of the IS link as Carlos' behaviour did not indicate he was attempting to inflict maximum casualties, given that he was armed with an M4 rifle but did not shoot at people who were fleeing the scene, instead firing warning shots into the air.
"I would call him a habitual gambler. He lost everything in his life and became crazy. He lost his mind. He vented his anger [through the incident]," President Duterte told reporters on Sunday, as cited by AFP.