The FBI is studying a report that the four terrorists who seized the plane may have been attempting to steer it towards a cluster of nuclear power stations on the east coast of America. The most likely target was Three Mile Island, site of America's most serious nuclear accident in 1979.

The 103 nuclear power reactors in the U.S. are vulnerable to acts of terrorism and the Nuclear Regulatory Commission and other government agencies must impose stricter safety measures to protect the facilities, two watchdog groups said September 25, 2001. The Washington-based Nuclear Control Institute and the Los Angeles-based Committee to Bridge the Gap urged the government to immediately use National Guard troops to protect nuclear plants from attack. The Three Mile Island nuclear power plant in Goldsboro, Pennsylvania is seen in this March 22, 1999 file photo. (Tim Shaffer/Reuters)

United Airlines flight 93 crashed into a field near the tiny town of Shanksville, in Pennsylvania, 90 minutes after taking off from Newark, New Jersey. All 44 passengers and crew on board died.

Until this weekend it had been assumed that the hijackers of the plane, a Boeing 757, were planning to fly it either to the presidential retreat at Camp David, or to Washington and crash it into the White House or the Congress and Senate buildings on Capitol Hill. But security officials have now revealed that within a week of the attacks, the FBI sent a report to MI5 saying that a "credible source" had said that the terrorists might have been planning to hit a nuclear plant.

Had it breached the plant's reactor vessel, such a strike could have caused an incident on the scale of the Chernobyl nuclear plant in Ukraine, which spread radioactive material over thousands of square miles in 1986.

US security sources say that Three Mile Island, which is part-owned by British Energy, was the subject of surveillance by some of the hijackers and their associates in the months before the terrorist attacks. One security official said: "Early on in the investigation we did receive a report from the FBI that the plane may have been heading for a nuclear power station. This was based on their analysis that Pittsburgh is near several power stations.

"There is some plausibility to this and we're not trying to dismiss it. But it may well be that nobody will ever know where the plane was going."

The "nuclear meltdown" assessment has not been independently confirmed but was taken seriously enough by the FBI to pass to European governments, including Britain and France.

The analysis is based on a study of flight 93's flight path and the fact that there are five nuclear power stations in the area. Experts say that the plane does not appear to have been hijacked until it was passing over West Virginia, some 200 miles beyond Washington. It then made a series of sharp turns before going into a steep descent. Aviation experts say that at this point there were three nuclear power stations between the plane and Washington and directly in its line of flight: Three Mile Island, Peach Bottom and Hope Creek.

Investigators cannot understand why the plane would have descended so early, unless its intended target was much nearer than Washington. The descent could have been an error by one of the hijackers, but if so, they cannot understand why the plane did not then climb again once control was regained.

America has since tightened security around nuclear stations and has taken steps to withdraw maps on the internet showing the location of nuclear plants. A French government minister said last week that fighters would shoot down aircraft heading for its nuclear plants. A missile defense system had been positioned at the Le Havre nuclear reprocessing plant.

In Britain, security around all nuclear sites has also been increased. David Blunkett, the home secretary, has given new powers to the 500-strong police force that guards the sites. Atomic Energy Authority police will be able to patrol an extra 13 civil nuclear sites, including Sizewell, Hinkley Point and Dungeness.

Engineering experts are divided over whether concrete containment shields around nuclear power stations could withstand a direct hit from a large passenger aircraft, especially one carrying 200,000lb of fuel, as was flight 93, enough to reach its destination of San Francisco.

The containment buildings generally have an outer structure, which for much of the dome is 3ft-thick concrete containing large amounts of reinforcing steel. Inside is a steel "lining" 1in-4in thick.

There are usually two more concrete walls close to the reactor, each 1ft thick and with reinforced steel bars. But these walls do not enclose the top of the reactor completely. The reactor vessel itself is about 4in-6in thick and made of high-carbon steel.

All reactors are designed to withstand impact by a light plane. Experts say it is unclear whether a larger modern jet loaded with fuel, deliberately flown at high speed, could break open the reactor vessel. The resultant fire could, however, cause enough damage to allow radioactive material into the air.

The drama aboard flight 93 as a small group of passengers tried to seize control of the plane from the hijackers during its final few minutes has become an emblem of American heroism during the events of September 11.

Delayed 40 minutes in taking off from Newark's congested airport, the plane was in the early stages of its journey when its passengers started hearing that other aircraft had been hijacked and at least one had flown into the twin towers of the World Trade Center.

Todd Beamer, one of the passengers, called an emergency operator on an onboard telephone after he and fellow passengers learnt of the first attack. He explained that flight 93 had also been hijacked. He said there were three hijackers - two with knives and one with what he thought was a bomb strapped to his waist. In fact, there were four, and by this time the fourth was almost certainly flying the plane.

Beamer, who was married with two young sons, told the operator: "We're going to do something. I know I'm not going to get out of this." He explained that some of passengers had decided to jump on the terrorist thought to have the bomb.

With the telephone left on, he could be heard saying: "Are you guys ready? Let's roll." The operator heard screams and a few minutes later the line went dead.