Murder charges have been filed against Juan Duntugan, 25, who admitted he bludgeoned a Peace Corps volunteer with a rock and a wooden stick in a fit of rage when she bumped into him during a solo hike last month. Associated Press   Peace Corps volunteer Julia Campbell, a former Punahou student, was found buried in a shallow grave last month in the Philippines. Associated Press

The charges against Juan Duntugan were filed Thursday in Banaue, in northern Ifugao province, where Julia Campbell was found buried in a shallow grave last month, assistant provincial prosecutor Marvin Ngayawan said.

Campbell was a former Hawai'i resident who attended Punahou School from 1979-82 while her family lived in 'Aikahi Park.

Previously prosecutors were treating the case as a homicide, which carries a maximum 20-year prison sentence. Murder is punishable by up to 40 years because it can involve treachery or cruelty.

Campbell, 40, disappeared April 8 during a solo hike to Ifugao province's famed mountainside rice terraces.

Duntugan, 25, admitted he bludgeoned Campbell with a rock and a wooden stick in a fit of rage when she bumped into him.

Campbell, of Fairfax, Va., was to leave the Philippines in June after serving two years with the Peace Corps.

Duntugan's confession showed he attacked Campbell from behind, Ngayawan told The Associated Press by telephone.

"There was treachery. Clearly Campbell ... could not defend herself," he said. The autopsy revealed "additional and unnecessary beatings," indicating cruelty.

Her body was found April 18 when soldiers found one of her feet protruding from the shallow grave after it was apparently dug out by a dog. Duntugan surrendered to police nine days later.

Campbell, a former journalist who worked for the New York Times and other media organizations, came to the Philippines, in March 2005.