Lascañas has made a U-turn from his previous Senate testimony when he denied the existence of the so-called Davao Death Squad and the allegations of confessed DDS hitman Edgar Matobato.
He had testified at the inquiry last October after he was implicated by another witness, Edgar Matobato, a former militiaman who said Duterte ordered him and others to kill criminals in gangland-style assaults that left hundreds of people dead.
Death squad allegations
However, this week, in Manila, Philippines the retired policeman told media at the Senate that Duterte had ordered the killing of several people when Mayor of Davao City.
Among those he said were killed included his own brothers, a pregnant woman, her young son and radio commentator Jun Pala who was critical of the Duterte. he also said that orders were given to bomb mosques in the early 1990s.
Human rights lawyers
Lascañas appeared with human rights lawyers Arno Sanidad, Arnold Padilla and Jose Manuel Diokno and Sen Antonio Trillanes IV.
Mr Sanidad said that he believed these revelations were possible grounds for impeachment of Duterte.
Duterte has denied his administration backs unlawful killings of suspects under his deadly crackdown against illegal drugs that is feared to have killed more than 7,000 mostly drug users and petty drug pushers since he took office in June.