An officer was shot on Tuesday morning at the Presidential Security Group (PSG) Complex inside Malacañang Park.
Presidential Spokesperson Ernesto Abella said the executive branch was confident that the Ombudsman would conduct an impartial investigation on Duterte's reported multi-billion peso networth.
Washington has said it supports the country's anti-drug efforts but has also urged Manila to respect human rights and the rule of law in its operations.
An official from the presidential palace, who asked not to be identified because he was not authorised to speak to the media, told reporters Duterte was not there at the time. "Certainly no foul play". He died from a single gunshot wound to the chest.
Trillanes, a staunch critic of Duterte, has repeatedly accused the President and his family of pocketing money collected from illegal activities and funneling it into several bank accounts. Aquino said that the unit has been investigating the shooting. He was due to attend some events in Manila on Tuesday evening.
Duterte is notoriously unpredictable and often causes confusion with sudden changes to his crowded public schedules, which typically start in the afternoon and can run as much as four hours late, often because of his long, unscripted speeches.
According to a report in Inquirer.net, Major Harim Gonzaga, 37, was found dead inside his office by his wife at around 9am today, PSG chief Lope Dagoy said.