Jittery world oil markets opened Tuesday with strong price increases in the first significant trading since a terror attack targeting Western oil workers left 22 people dead.

Saudi authorities, meanwhile, were still searching for three Islamic militants involved in the weekend shooting rampage and hostage standoff that has left foreign businesses reconsidering whether to stay in Saudi Arabia (search).

Police responded to numerous tips since the attackers fled the upscale Oasis residential compound Sunday. One — said to be the ringleader — was wounded and arrested at the compound.

Brent crude oil prices for July delivery were up $1.82 a barrel, at $38.40 and rising, in Tuesday trading on the International Petroleum Exchange (search) in London. Light crude, in after-hours electronic trading in New York, also was up more than $1, to $41.07 per barrel over Friday's close. Both markets had been closed Monday for holidays, though some Asian markets also had risen in a reflection of concern.

The attack came just ahead of an Organization of Petroleum Exporting Countries (search) meeting Thursday in Beirut, where ministers begin debate on a possible production increase to try to bring already high prices down.

Unease over the second terror attack in a month targeting oil-related interests in the kingdom also left foreign companies and expatriate workers re-evaluating whether to stay. The question arises after every terror attack — and there have been several in the past year — and while some do leave, many generally choose to stay at their well-paying jobs.

The last two attacks, however, have been particularly brazen and brutal. On May 1, gunmen killed six people at a petrochemical compound in the Western industrial city of Yanbu, and dragged an American victim's body behind a car through the streets.

The latest attack evolved into a 25-hour siege, with gunmen hunting down non-Muslim foreigners, reportedly mutilating some of their victims.

The U.S. Embassy had urged Americans to leave Saudi Arabia even before the Yanbu attack, which prompted some departures. Now, the British government is warning more attacks are imminent.

On Tuesday, an official with a Japanese government-backed oil research group told Dow Jones Newswires it may evacuate 15 researchers from Saudi Arabia — 11 of whom lived at the Oasis compound — because of security fears. The Japan Cooperation Center, Petroleum is consulting with the Saudi Arabian Oil Co., or Aramco.

"We will soon dispatch our officials to Aramco's office and determine safety measures for the 15 researchers," an unidentified institute official told Dow Jones. All the researchers are Nippon Oil Corp. employees taking part in a joint project with Aramco.

A spokesman for the Japanese External Trade Organization in Riyadh said four or five companies are sending relatives of employees home "to be on the safe side." He cited Nippon, Sojitz Corp. and Idemitsu Kosan Co. as among them.

The spokesman, talking on condition of anonymity, said it was premature for companies to consider leaving altogether. The Japanese Embassy has not formally advised its 700 citizens to leave, but an embassy official told the AP it is asking Japanese nationals if they want to go.

Some Westerners working in Saudi Arabia were considering nearby Bahrain, where many already live, a half-hour commute from the Khobar area over the 18-mile King Fahd Causeway.

A Bahrain real estate agent, speaking on condition of anonymity, said two Westerners from Khobar came to her Monday looking for houses ready for occupancy following the attack.

A board member of a private school in Bahrain, who also insisted on anonymity, said two Khobar-based multinational companies had contacted the English-language school before the attack because they were considering relocating outright and were checking for spots for their employees' children.

Bahrain, a close ally of the United States, has not seen the kind of extremist violence that has erupted repeatedly in Saudi Arabia in the past year. Unlike Saudi Arabia, it has nightclubs, movie theaters and bars and allows women to drive and appear in public unescorted and in Western-style dress.

Saudi authorities, meanwhile, hunted for the three suspected al-Qaida militants who used hostages as human shields to escape the Oasis compound. The attackers fled to nearby Dammam, where they abandoned their truck for a car commandeered at gunpoint, a police official said Monday.

In the evening, several police cars surrounded a mosque next to a McDonald's restaurant in Khobar after police got word that suspected terrorists were inside the mosque. A policeman on the scene said two people had been detained on suspicion of terrorist ties.