Rescuers racing against the clock in a desperate attempt to find survivors in the World Trade Centre's rubble faced a new obstacle yesterday - rain.

Three days after hijacked passenger jets ploughed into the twin 110-storey skyscrapers, thunder and lightning brought a torrent of rain to the mammoth heap of ash and twisted wreckage.

But sodden rescue workers kept at it during the downpours.

"There's no question they're hampered by it," said New York City's Mayor Rudolph Giuliani. "But there's still a strong hope we'll be able to find people, recover people and save them."

Meanwhile, at an armoury, in hospitals and on the streets of Manhattan, thousands of distraught families searched for the missing.

Almost every sentence began the same: "Have you seen . . ." And nearly every plea ended the same: "If you know anything, please call . . ."

More than 2,500 people stood in line at the armoury on 26th Street and Lexington Avenue, waiting to complete missing-persons reports.

At St. Vincent's Hospital, where many of the victims from Tuesday's World Trade Centre attacks were taken, relatives waited to find out if their loved ones had been admitted.

Others stood on the street, trying to persuade reporters to print or broadcast the names and photographs of relatives they could not find.

Identifying some of the victims could take years, and in some cases they may never be identified, a British expert said yesterday.

Professor Peter Vanezis, of Glasgow University, who is assembling a team of scientific experts to help, compared the scale of the disaster with the wartime London blitz.

The professor, a forensic pathologist who has worked at war crime atrocity scenes in Kosovo and at numerous air crashes and other disasters, said merely identifying the victims would not be enough for experts working at the American crash scenes.

They would also be trying to identify the role each victim played in events, whether they were on the ground or on the planes, and if they showed any knife or gunshot injuries.

"From our point of view, working in the mortuary, we have to assume that people in an aeroplane have been traumatised in a physical way, other than by the crash itself," he said.

"We would need to be looking, for example, to see if they had stab wounds, gunshot wounds.

"We would need to identify the people who were the terrorists, who were the key players on the particular aeroplane.

"To do that helps in the reconstruction of what happened."