A shipment of nuclear fuel has arrived in Russia after a top-secret international operation to remove it from Serbia, where it was feared terrorists could seize it to make a nuclear or dirty bomb.
In the dead of night, armed men in balaclavas surround a long convoy of trucks in the woods just outside Belgrade. Radios crackle as they prepare for a long journey.
Their mission is to escort a dangerous cargo, the kind terrorists would dearly like to get their hands on.
Inside blue, bomb-proof, fire-proof containers on the trucks are 2.5 tons of radioactive material, including 13kg of highly enriched uranium that could be used for a nuclear weapon.
This is the largest shipment of its type ever made, and will clear Serbia of all its civilian highly enriched uranium.
Just before two in the morning, the president of Serbia, Boris Tadic, sweeps in.
"We have significant security here," he tells me. "This is extremely important."
Two hours earlier I had been taken into the decommissioned reactor building where the Soviet-origin nuclear material had been stored.
During the Cold War, the US and Soviet Union both provided countries with reactors to carry out research. Some, like that here in Vinca, ran on highly enriched uranium which came from the USSR.
Most of Vinca's highly enriched uranium was removed in 2002, but the remaining enriched uranium and large amounts of spent fuel were still kept here, in often poor conditions.
Yellow tape seals off corridors and wires hang loose. Security has been improved in recent years with new access controls and CCTV cameras, but this site was still top of the global worry list. Around the world there is a race to secure vulnerable material before terrorists can get their hands on it.
At 0200, the long, snaking convoy heads out of the gate.
About 3,000 Serbian police have been drafted in for the operation. The roads have been cleared of traffic and police line the route.
With a helicopter buzzing overhead, the convoy heads north toward Subotica near the Hungarian border. It arrives at dawn and the containers are unloaded on to a train at a depot outside the town.
The next morning the train, with a few officials in a special carriage, heads over the border into Hungary where the security guards are changed.
Restrictions meant the material could not be flown and transporting it by land directly proved impossible because some countries did not want it to cross their borders.
Negotiating the route and gaining permission for the material to pass through countries that would accept it took five years of planning, and close co-operation between US, Russian officials and International Atomic Energy (IAEA) officials.
The Vinca fuel rods are considered particularly dangerous because of their handy size (about 15cm/6in long) and the fact that their radioactivity has lessened, making them easier to handle.
This makes them ideal for a so-called dirty bomb.
"You can put it in your hand. You can wrap a stick of dynamite around it and you can put it in your backpack or purse," explains John Kelly of the IAEA.
"And you can create a disaster in just about any city."
The train pulls into the Slovenian port of Koper at dawn the next day. Heavy rain falls as the containers are loaded on to a ship.
There will be no armed guards on board, but special security precautions are being taken, according to Sergey Naletov of the Aspol Baltic shipping line.
"There is special equipment installed on board the vessel to trace its location," he explains as we are shown round the ship.
Once it departs, a US official shows me how he can monitor its exact position using software on his phone.
Officials also hint that special measures are taken to track the boat in international waters, but are unwilling to say exactly what these are.
The ship travels through the Mediterranean, past Gibraltar and through the English Channel on its way to the Russian Arctic port of Murmansk.
In all, the containers will have made a journey of close to 11,300km (7,000 miles) taking over a month.
In Murmansk, the material is unloaded on to a train to be taken on the final stage of its journey to the Mayak reprocessing facility in the Urals.
In 2009, President Obama announced an international effort to secure all vulnerable material in the following four years.
"This is the sixth country we've cleared out of all highly enriched uranium since President Obama's speech," explains Sarah Dickerson of the US National Nuclear Security Administration who followed the material through from Vinca to Koper.
"There's a lot of work that still remains to be done."