SAN'A, Yemen - Shepherds found the mutilated bodies yesterday of two German nurses and a South Korean teacher who were kidnapped while picnicking in an area of Yemen known as a hideout for al-Qaeda.

Experts said the killings bore the hallmarks not of local tribesmen but rather of jihadists who had returned home after fighting in conflicts in Afghanistan, Iraq, and elsewhere.

The dead women disappeared in the remote northern province of Saada on Friday while on an outing with six other foreigners: a German doctor, his wife, and their three young children, and a British man. The whereabouts of the six were unknown, Yemen's government said.

Authorities announced a state of high alert in the area and were "conducting extensive searches and investigations," according to a government statement.

The foreigners worked for World Wide Services Foundation, a Dutch aid group helping with medical care in the province.

The incident is the latest attack against foreigners in this impoverished Arab nation on the tip of the Arabian peninsula where al-Qaeda has a firm foothold in its remote areas.

The government blamed a Shiite rebel group that has been leading an uprising in the province for several years, but the group denied it had anything to do with the kidnappings. Initially, Yemeni security officials had reported that all nine foreigners were killed, but the government later said six were still missing.

Nearly all past fatal attacks against foreigners in Yemen have been by Islamist militants.

"I think that it would have to be outside sources" that carried out the attack, said Magnus Ranstorp, a terrorism expert at the Swedish National Defense College, noting that the killings, including reports of mutilation, bear the hallmarks of al-Qaeda.

The killings "represent a nasty turning point in Yemen," he said.

In the past, tribesmen have kidnapped foreigners to wrest concessions on local issues from the government - including ransoms and the release of jailed relatives. But they usually treated hostages well and released them unharmed. Past abductions by al-Qaeda, however, have ended with hostages' deaths.