PRESIDENT Rodrigo R. Duterte’s approval rating fell in September, amid a Senate investigation of the country’s jail system for corruption that allegedly leads all the way to top police officials.

The president got an approval rating of 78%, 7 points lower than a quarter earlier, according to the latest poll by Pulse Asia Research, Inc. His trust rating also dropped by 11 points to 74%, the polling firm said in an email.

His latest ratings come as senators investigate money-making schemes inside the national jail in Muntinlupa City. The Bureau of Corrections had illegally freed about 2,000 felons for good conduct even if they were ineligible because they were convicted for various heinous crimes.

Mr. Duterte was forced to fire his prison chief, Nicanor E. Faeldon, whom he had retained in several government posts after his stint at the Bureau of Customs.

The Ombudsman has suspended dozens of jail officials for neglect of duty, among other things.

Still, the president retained majority approval ratings across all geographic areas and socioeconomic classes — 69% to 92% and 75% to 84%, respectively, Pulse Asia said.

“Trust is the prevailing sentiment in the different geographic areas and socioeconomic groupings toward President Duterte (62% to 91% and 71% to 80%, respectively),” the pollster said.

Meanwhile, Vice President Maria Leonor G. Robredo’s approval rating also fell by 5 points to 50% in the third quarter, while her trust rating dipped by 6 points to 46%, according to Pulse Asia.

Senate President Vicente C. Sotto III’s approval rating fell by 5 points to 72%, while his trust rating dipped by 7 points to 66%.

Chief Justice Lucas P. Bersamin’s approval rating rose by a point to 42%, while his trust rating dipped by a point to 34%, according to the September poll.

Speaker Alan Peter Cayetano, who assumed his post in July, got a majority approval rating of 64% and a trust rating of 62%, Pulse Asia said.

The firm talked to 1,200 adults for the poll, which had an error margin of ±2.8 points.