MANILA, PHILIPPINES—Philippine troops have found the remains of a German hostage who was beheaded by Abu Sayyaf militants in the country’s south, vowing Sunday to rescue more than 30 other captives and crush the ransom-seeking extremists.

Marines dug up the head and body of Jurgen Gustav Kantner late Saturday in the mountainous hinterlands of Indanan town in Sulu province, where the militants are holding at least 31 other foreign and Filipino hostages, said regional military commander Maj. Gen. Carlito Galvez Jr.

The 70-year-old Kantner was seized from his yacht with his female German companion off Malaysia’s Sabah state in November. Kantner’s companion was fatally shot on the yacht, which was later found in the southern Philippines, according to the military.

Read more: Filipino militants release video of beheading of German hostage

The couple had survived a kidnapping ordeal off Somalia in 2008.

“Once again, the command is sending its deep regrets to the family for not being able to rescue Mr. Kantner on time,” Galvez said. He repeated a pledge to rescue other hostages and crush the Abu Sayyaf.

President Rodrigo Duterte’s spokesman, Ernesto Abella, said the government “will leave no stone unturned in squarely addressing the evils of extremism and plain banditry.”

“Rest assured these mindless acts will not go unpunished,” Abella said.

German Chancellor Angela Merkel has condemned Kantner’s killing as an “abominable act.” The Abu Sayyaf circulated a video of the beheading online.

Duterte has said Filipino forces tried their best but apologized to Germany and Kantner’s family after troops failed to rescue him in his nearly four months of jungle captivity in Sulu, a poor Muslim province 590 miles (950 kilometres) south of Manila.

About two dozen Filipino troops were wounded in clashes that also killed 16 Abu Sayyaf gunmen in efforts to rescue Kantner. After he was beheaded, troops intensified ground assaults and airstrikes.

On Sunday, marines killed four Abu Sayyaf militants in an assault near Sulu’s Maimbung town. At least 10 other militants were killed in a separate clash Friday that also wounded 18 troops near Patikul town, said Sulu’s military commander, Col. Cirilito Sobejana.

An intelligence report seen by The Associated Press said the militants behind Kantner’s abduction and killing included Abu Sayyaf commander Hatib Hajan Sawadjaan and his nephew, Mujil Yadah, who was also allegedly involved in the 2015 kidnappings of a Norwegian, a Filipina and two Canadians from a yacht club in the south. The two Canadians were separately beheaded last year.

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According to the report, the other kidnappers of the German included Moammar Askali and Idang Susukan. Askali, a young militant, insisted that Kantner should be killed on schedule as they had threatened to do, but others wanted to wait longer to get a huge ransom, which was last pegged at 30 million pesos (about $ 797,966), the report said.

The Abu Sayyaf, which has more than 400 fighters, has been blacklisted by the Philippines and the United States as a terrorist organization for kidnappings, beheadings and bombings.