A riot broke out at a Philippine prison on Wednesday, killing one inmate and wounding three, all of whom had been expected to testify at a congressional inquiry on alleged criminal activity by the president’s biggest critic. Rolando Asuncion, chief of the Philippines’ corrections bureau, said a drug trafficker was stabbed to death in the brawl, and one of the three wounded men, kidnapper Jaybee Sebastian, had been scheduled to give evidence next week. Dates had not yet been set for testimony from the rest. It’s just a plain riot. This is just a usual occurrence in prisons. Rolando Asuncion, chief of the Philippines’ corrections bureau The inquiry, initiated by allies of President Rodrigo Duterte, will test allegations that his rival, Senator Leila de Lima, a former justice minister, was colluding with prison inmates and taking payments from them to fund her poll campaign. Asuncion dismissed a suggestion by House speaker Pantaleon Alvarez that the riot could have been an attempt to derail the inquiry, however. “It’s just a plain riot,” he told reporters. “This is just a usual occurrence in prisons.” Senators this month ousted De Lima from the leadership of a panel investigating a spate of nearly 3,000 killings during Duterte’s three-month-old war on drugs , squashing a rare domestic challenge to his high-profile crackdown. “We don’t want him dead,” Justice Secretary Vitaliano Aguirre told reporters, referring to the wounded Sebastian. “We’re counting on him to spill the beans, he goes direct to De Lima.” This Philippine senator is a vocal critic of Duterte’s drug war - and she’s been ousted from the killing inquiry she started The killings have drawn widespread international criticism, including from the United Nations, drawing angry responses from Duterte. But De Lima’s removal, and the subsequent inquiry, show how much clout Duterte has won during his short time in office. Duterte’s war on drugs in the Philippines can’t hide his policy incompetence At a news conference, De Lima expressed anger at what she called the “mafia tactics” being used to get prisoners to cooperate in the case against her. Duterte, about to leave on a two-day visit to neighbouring Vietnam, urged his staunchest critic to take a rest. “If she continues yakking there, she would have a nervous breakdown,” he said.