Two Soviet-era nuclear generators which sparked a huge international hunt in Georgia may not exist, authorities admitted on Monday.
The Strontium 90 generators were believed to be hangovers from the Soviet military presence in Georgia.
Dozens of experts took part in a two-week search of 550 square kilometres (200 square miles) of land in the west of the country, some of it so remote that they had to travel on foot or on horseback.
But now officials at the International Atomic Energy Agency say new evidence has come to light which suggests the generators may not be there.
Another search planned for later this year will focus on looking for other materials, Mark Gwozdecky, the IAEA's public information director, told BBC News Online.
"It was supposed there were two generators out there, but it is not clear now whether they are there to be found," he said.
The nature of the new evidence, or when it came to light, is not being disclosed, but Mr Gwozdecky insisted it had been right to press ahead with the search.
"We felt there was certainly a strong possibility (of finding them)," he said. "It was something that needed to be done."
"The search was completed and people in that area can be reassured."
Experts say will return to Georgia in September to search another part of the country for nuclear material, but will not be specifically looking for the Strontium generators.
Helicopters fitted with sensitive monitoring devices will criss-cross the search area, looking for sources of radiation.
Three minor sources of radiation were found during the first search, but Georgian officials stress they were far to weak to have been the Strontium generators.
The search was conducted by 80 experts from Georgia, India, France, Turkey and the US.
Four months earlier, two Georgian forestry workers suffered severe radiation sickness and burns when they found other Strontium 90 sources abandoned in woodland.
The men are still being treated in France and Russia for the injuries they suffered.
The generators would have been used to power communication stations in remote areas, and the IAEA describes them as "highly radioactive".
More than 280 other radioactive sources have already been recovered from Georgia since the mid-1990s. Some were from abandoned Soviet military bases.
Experts say abondoned nuclear materials abound in the former Soviet countries.
"It's incredibly important to keep searching," nuclear engineer John Large told BBC News Online. "But the IAEA does not have the resources to do this.
"You are talking about the accounting collapse of an entire superpower, and the Soviets were not good at paperwork."
Dirty bombs
Georgia was a centre for much of the Soviet-era nuclear research and development, he added, making the problem particularly serious there.
Many of the technicians themselves were Russians, who left Georgia after the collapse of the Soviet Union, making the region's radioactive materials particularly vulnerable to being "misplaced" or mishandled.
The IAEA has been working with the Georgian authorities since 1997 to try to recover missing material, and to upgrade safety.
Last week the IAEA revealed that substances to build a so-called "dirty bomb" laced with radioactive material could be found in almost every country in the world.
More than 100 countries may have inadequate control and monitoring programs necessary to prevent or even detect the theft of these materials, the International Atomic Energy Agency (IAEA) says.
Strontium 90 could also be used for this purpose, but there is no suggestion that the mystery generators of Georgia have fallen into the wrong hands.