For four decades, residents of the tiny Pennsylvania town of Kecksburg have told their story of strange blue lights in the sky one winter's evening and a fireball crashing into woods.

On 9 December, 1965, they say, they saw armed soldiers cordoning off the area and a large metallic acorn-shaped object bearing strange hieroglyphics driven off at speed on the back of a lorry. They talk of menacing plain-clothes officials visiting homes and warning local people not to tell anyone of what they saw.

Until now the US government has denied that anything sinister took place. It has maintained that a thorough search of the woods by the air force, the only federal agency to have acknowledged it was there, found nothing. But now Nasa has been ordered to examine its X-Files to solve the mystery.

Steve McConnell, Nasa's public liaison officer, has admitted two boxes of papers from the time of the Kecksburg incident are missing. The episode has parallels to the 1947 Roswell incident, when a UFO was said to have landed in New Mexico.

'For so many years, a lot of good people in Pennsylvania were told by their government that what they had to say was a lie or that they were hallucinating,' said Leslie Kean, a journalist who launched a lawsuit four years ago to force Nasa to open its archives.

Washington judge Emmett Sullivan refused to accept Nasa's claim that the papers had been lost. He gave it until the end of the year to examine its records. 'Something came down that night,' said Kean.

'Nasa has been stonewalling and now it's required to do the search it didn't do in the first place. It's a victory for those patriotic people who didn't like being told that they were making things up.'

Stan Gordon, a UFO investigator living close to the site, interviewed several witnesses. He said: 'It's interesting that [witnesses say] it was made of one solid piece of metal with no panels or rivets, and that it was moving relatively slowly and made almost a controlled landing.

'I have no doubt the government knows a lot more about this than it has revealed to the public.'