Arturo Lascanas admitted to lying in October during a Senate inquiry into alleged extrajudicial killings linked to Duterte, but said he did so only because he feared for his family's safety and because police had warned him to "deny everything".
He said he had personally killed 300 people, about 200 as a member of a "Davao death squad", with his last in 2015. He also detailed two cases where he had murdered critics of Duterte, under the instruction of the then-mayor's bodyguard.
Lascanas, who broke down in tears before the media when he revealed his story two weeks ago, is the second person to testify before lawmakers to Duterte's alleged links to a clandestine hit squad.
Duterte's allies dismiss the claims as a plot by his opponents to discredit a popular leader and his war on drugs, a campaign that critics say has disturbing similarities to a pattern of mysterious killings in Davao.
"I feared for the life of my loved ones," Lascanas said when asked why he had earlier denied the death squad existed.
He said he changed testimony because he was tormented by what he had done and wanted the truth to "set me free".
It was "because of my desire to tell all the truth, not only because of my spiritual renewal, but the fear of God, I wanted to clear my conscience", he said.
Duterte has repeatedly denied involvement in summary executions, either as president or during his 22 years as Davao mayor. His police chief Ronald dela Rosa, a former Davao police chief under Duterte, has dismissed the death squad claims as a myth created by the media.
Human rights groups have documented about 1,400 suspicious killings in Davao while Duterte was mayor and critics say the war on drugs he unleashed as president has the same hallmarks. Numerous investigations have found no proof linking Duterte to those deaths.
More than 8,000 people have been killed nationwide since Duterte took office eight months ago, mostly drug users killed by mysterious gunmen in incidents authorities attribute to vigilantes, drugs gang members silencing informants, or unrelated murders.
Police reject activists' allegations that they are behind most of those killings. Police say they were responsible for 2,555 of those cases, when drug suspects had resisted arrest.
On Monday, dela Rosa announced the re-launch of police anti-narcotics operations after a month-long suspension of police involvement in the campaign.
Reported and written by Martin Petty