His former hit-man, Edgar Matobato, told a senate hearing into Duterte's ongoing anti-crime crackdown that the then Davao city mayor had criminals thrown in the sea after having their stomachs slashed so they wouldn't float to the surface.
Matobato also claimed to have overheard Duterte order the bombing of mosques in Davao as retaliation for an attack on a cathedral.
"Our job was to kill criminals like drug pushers, rapists, snatchers," said the 57-year-old, who admitted to killing more than 50 people while working for a "Davao Death Squad".
"They were killed like chickens," he told the televised hearing.
Rights groups have documented some 1,400 suspicious killings in Davao since the early 1990s and critics say the bloody war on drugs Duterte has unleashed since taking office on June 30 bears the same hallmarks.
In a series of controversial speeches during his election campaign he appeared to encourage extrajudicial killings.
More than 3,500 people, or about 47 per day, have been killed in the past 10 weeks, some 58 percent by unknown assailants and the rest in legitimate police operations, according to police.