MANILA — A news radio host who had been a prominent critic of President Ferdinand Marcos Jr. of the Philippines was fatally shot in his car during an ambush near his home, the authorities said on Tuesday.

The journalist, Percival Mabasa, was killed on Monday night outside the capital, Manila, by two men on motorcycles who later escaped, said Brig. Gen. Roderick Augustus Alba, a spokesman for Philippine National Police. The shooting occurred in the suburb of Las Pinas, outside the gated community where Mr. Mabasa lived.

A manhunt was underway Tuesday as the authorities investigated the killing, Mr. Alba said. Officials did not give a possible motive.

Mr. Mabasa was the second journalist to be killed in the country since President Marcos, the son of the dictator Ferdinand E. Marcos, took office in late June after a polarizing election, according to the National Union of Journalists of the Philippines. Last month, the radio broadcaster Renato “Rey” Blanco was stabbed to death in the central Philippines, hundreds of miles south of the capital. A suspect in that case later surrendered to the police, but no charges have been filed.

Former President Rodrigo Duterte had threatened journalists who documented his violent antidrug campaign with physical violence. At least 23 journalists were killed during Mr. Duterte’s six-year presidency, according to rights organizations and the national journalists’ union.

Mr. Mabasa, known as Percy Lapid to his followers, had accused top Philippine officials of corruption in the hard-hitting radio program that he hosted for years in Manila.

Among his targets were Mr. Duterte’s antidrug campaign and perceived attempts by supporters of the Marcos family to distort history by portraying the elder Marcos, who died in 1989, as a victim of his political enemies. (In fact, Ferdinand E. Marcos was a dictator who tortured and jailed opponents and was accused of looting as much as $10 billion from the government.)

In recent weeks, Mr. Mabasa had criticized the current Marcos government for what he said was corruption involving anomalies in sugar imports through a state agency. The president’s executive secretary, Vic Rodriguez, resigned last month after the backlash generated by Mr. Mabasa’s reporting.

The victim’s brother, Roy Mabasa, a fellow journalist and a former president of the National Press Club, said in an interview on Tuesday that there was no lead in the murder because police officers were still gathering CCTV footage, including from the cameras that his brother had mounted on his car.

“But knowing my brother, those behind his killing are the persons he has been hitting on his show,” he said. In a separate statement, Mr. Mabasa’s family called for justice and said that his “fearless” reporting had helped to dispel fake news proliferating on social media in the Philippines.

“We strongly condemn this deplorable crime; it was committed not only against Percy, his family, and his profession, but against our country, his beloved Philippines and the truth,” the family said.

Former Vice President Leni Robredo, who ran unsuccessfully against Mr. Marcos in the recent presidential election, called Mr. Mabasa’s death a “huge loss at a time we are up against brazen lies spread in public.” While she did not elaborate, experts have attributed Ms. Robredo’s loss partly to fake news spread by her political opponents through social media.

“In a truly free society, there should be no space for violence against our journalists,” she said.

The national journalists’ union also condemned the killing, saying that it underscored how journalism remained a dangerous profession in the country.

One of the Duterte administration’s frequent targets was Maria Ressa, the country’s most prominent journalist and the founder of news site Rappler. Two years ago, she and a colleague were convicted of cyber libel by a court in Manila. Last year, Ms. Ressa won the Nobel Peace Prize, in part for Rappler’s coverage of Mr. Duterte’s violent antidrug campaign.

Jason Gutierrez reported from Manila and Mike Ives from Seoul.