According to Joe Arpaio, Maricopa County Sheriff, typewritten notes were found at the scene of the accident, referring to the sieges of Waco in 1993, and of Ruby Ridge, Idaho, a year earlier - both of which were rallying issues for anti-government fringe groups, and whose revenge may have been a motive for last April's deadly Oklahoma City bombing. The notes were signed "Sons of Gestapo," Mr Arpaio said, adding he believed the incident was the work of domestic terrorists.
Amtrak's prestige Sunset Limited train came off the tracks in wild and inaccessible country about 60 miles south- west of Phoenix at around 1.30 in the morning. It was on the way from Miami to Los Angeles, and had 248 passengers and 20 crew members aboard.
"There was a loud roar and everything was shaking," said one passenger. "It was beginning to tilt and then everything just shook and stopped." Another described the experience as "like a rollercoaster". In all, eight of the train's 12 carriages left the rails.
Investigators found that, in one section of track, 29 spikes which held the rails to the wooden sleepers had been removed, while an alarm system that would normally have warned the train's crew had been deliberately disconnected.
Also, police officials said, an electric wire tied to the track section prevented on-board computers from sensing anything was amiss. Moments before disaster struck, the Sunset Limited's driver had seen something on the track and had tried desperately to halt the train in time.
Reinforcing the theory of a link to the Oklahoma City attack was the mention, on another note discovered by a passenger after the crash, of the Bureau of Alcohol, Tobacco and Firearms (ATF), the federal agency which carried out the original raid on the cult-leader David Koresh's headquarters at Waco on 28 February 1993.
Also, it was pointed out last night, the remote Western Arizona desert is an area favoured by far-right loners and drifters, among them Timothy McVeigh, the prime suspect in the Oklahoma City bombing - the worst-ever terrorist attack in the US, in which 168 people were killed. Last night, FBI agents were joining the Amtrak investigation.
Rescue efforts were complicated by the difficult terrain, which is rocky and criss-crossed with ravines and gorges. Only four-wheel drive vehicles could reach the spot, and the injured were ferried by helicopter to Phoenix, where a shopping mall was turned into an improvised landing site from which they could be transferred to a local hospital.
Of the 100 victims, 12 were said to be in serious condition.