A sophisticated device with two separate detonating systems is believed to have triggered the bomb that brought down Pan Am Flight 103, killing 270 people, The Times of London reported Friday.

The newspaper said crash investigators believe the first trigger was a barometric device set off by altitude. This activated the second trigger, an electronic timer, to make the bomb explode an hour later, it said.Terrorists developed the double detonator technique after some airports started putting cargo through pressure chambers that would detonate pressure-activated bombs, The Times said.

The Frankfurt airport, where the Pan Am flight originated aboard a different plane, uses the pressure chambers, The Times said. The Times was first to report Wednesday that investigators concluded a bomb was responsible for the crash. The formal announcement was made hours later.

The Boeing 747 crashed Dec. 21 in Lockerbie, Scotland, en route from London to New York. All 259 people on board were killed, and 11 people on the ground were missing and presumed dead. So far 241 bodies have been recovered.

An anonymous male caller claiming to represent the Guardians of the Islamic Revolution, which last week said it blew up the plane, telephoned American news agencies in London Friday and threatened more action.

He repeated the claim of responsibility and told The Associated Press that unless the United States deports Reza Pahlavi, son of the late Shah of Iran, "there will be another present in the New Year for America."

Officials discounted his claim and reportedly were focusing on Palestinian and Iranian extremist groups. British Foreign Secretary Sir Geoffrey Howe, meanwhile, appealed to Middle Eastern governments to cooperate with the investigation.

As a worldwide hunt expanded, passengers using American airlines at four British airports underwent questioning Thursday and had their baggage X-rayed in accordance with Department of Transport orders.

In Washington, the Federal Aviation Administration ordered visual or X-ray inspection of all checked bags on U.S. airlines at 103 airports in the Mideast and Europe.

FAA Administrator Allan McArtor said the change "far exceeds international standards" and is based on anti-terrorist practices of the Israeli airline El Al.

Officials said the FAA is rushing delivery of devices that are capable of detecting all known explosives, including plastic explosives.

Check-ins proceeded smoothly at London's Heathrow Airport, but passengers were quizzed about who packed their bags, when they were packed, whether the bags had been out of their sight and whether strangers had given them parcels.

Their checked-in baggage was fed through X-ray machines and then bound with a tough plastic strip to prevent them from being opened en route to the planes. Some luggage was also hand-searched.