By Jonathan Fildes
Science and technology reporter, BBC News
Given pride of place in an unassuming museum on the East Coast of America is a pair of 200-year-old duelling pistols shrouded in mystery.
The intricately decorated guns were said to have been forged from the iron of a meteorite.
They were a unique gift from the commander of a South American region, which would later become Argentina, to the fourth US president, James Madison.
"Permit me therefore to present to your Excellency... a specimen of the first essays of the manufacture of arms established in the provinces of Buenos Ayres and Tucuman," wrote General Ignacio Alvarez in an accompanying 14-page letter.
Over time, they passed into the hands of Madison's successor - James Monroe - and are now on display at a museum dedicated to him.
Since that time, the story of their origin has gone unquestioned.
Now, scientists armed with a battery of hi-tech machines have probed the pistols in unprecedented detail.
Their findings cast doubt on the accepted theory of their origins and have thrown up a whole new set of questions for historians about the guns and the motives of the original protagonists.
"It's made the mystery even more mysterious," Meghan Budinger, curator at the James Monroe Museum and Memorial Library, told BBC News.
To uncover the secrets of the pistols, they were sent on a world tour of some of the most advanced microscopes and scanners.
HOW ISIS WORKS
Neutrons blasted at sample
Neutrons created by accelerating protons to near light-speed
Pulses of protons smash into block of tungsten, dislodging neutrons from the nuclei of its atoms.
20,000 trillion neutrons dislodged each second
Neutrons channelled into beams and directed at sample of interest
Neutrons bounce off the molecular structure and collected by sensors which record position and energy
Scattering tells scientists about how target atoms are organised
They underwent X-ray fluorescence to reveal their surface detail, and were passed through a CT scanner to reveal their inner workings.
But the key test - whether they were forged from a lump of metal from space - was performed at the ISIS neutron source in Oxfordshire, UK.
The colossal machine is able to probe matter at the atomic level, giving scientists unique insights into the structure and make-up of materials.
"We give a different picture of what the world looks like," said Professor Andrew Taylor, director of ISIS.
The facility, owned and operated by the UK's Science and Technology Facilities Council (STFC), is currently being expanded to add the so-called Second Target Station, which will give the super-scope even more capability.
It is more commonly used by scientists designing and testing new materials, such as those used in the Airbus A380 and the turbine blades found in Rolls Royce jet engines.
"We didn't set out to build a tool for archaeology," said Professor Taylor.
However, its abilities are ideally suited to probing historical artefacts.
"What my neutron beams tell you are where atoms are and what atoms do," said Professor Taylor. "We try to understand at a microscopic level the structure, arrangement and forces that hold materials together."
Crucially, this process is non-destructive.
"Without [ISIS], we'd have to take a hacksaw and cut chunks out of the artefact to look at under the microscope," explained Dr Evelyne Godfrey, who carried out the study.
The machine was used to compare Monroe's pistols to a fragment of a meteorite from the Campo del Cielo crater in Argentina; the supposed origin of the metals from which they were forged.
ISIS has been used to test and develop materials for the Airbus A380
The results were conclusive.
"They were completely different," Dr Godfrey told BBC News. "There were differences in microstructures, there were differences in carbon content, there were differences in chemical composition.
"We can say for sure they weren't made from meteoritic iron."
The results came as a blow to the museum.
"The results weren't quite what we were hoping," said Ms Budinger, diplomatically.
But, whilst the tests closed one chapter for the pistols, it opened another, packed full of new questions.
Curators of the objects had always assumed that the handles of the pistols were made of silver, but ISIS showed that this too was not the case.
"They were brass - and it was a funny type of brass, too," explained Dr Godfrey.
The closest match was a distinctive alloy exported from Southeast Asia at around the time.
"It looks just like silver but is cheaper - so they were making tableware and replacing silver in other objects," said Dr Godfrey.
Other tests revealed that the guns were not just ceremonial - as previously thought - but were fully functioning arms.
"It brings up all kinds of questions," said Ms Budinger.
"What exactly was General Alvarez's motivation? He wrote this very flowery letter saying how much he admired the United States and how much he admired Madison but then he gives them a gift that was not where he said it was from and it was made from cheaper materials."
There is a possibility that the General was duped - that he was told the pistols were made of meteoritic iron and gifted them in good faith.
"He may not have known what he was giving to Madison - at the time there would have been no way to prove it one way or the other," said Ms Budinger.
The partially solved mystery could stop there; but the research team have one further avenue to explore.
A third pistol is mentioned in General Alvarez's letter that was also supposedly forged from Campo del Cielo iron.
"We're trying to track that down," explained Ms Budinger.
If it is located and can be put through the same battery of tests as the first pair it could finally give historians clues to the real origin of the pistols.
"If it looks exactly the same that tells us that all three pistols were manufactured by the same person and that either General Alverez was duped or that it means he was lying and that none of the pistols were made from the Campo del Cielo crater.
"If the third pistol is different entirely then I think we have fairly good evidence that our pistols may not be the Madison pistols at all," said Ms Budinger
"In which case it's a whole new mystery."