Police are combing fields and towns in south-east France in a search for four thieves who stole dozens of kilograms of gold from an armoured van before setting cars on fire near a major motorway.
The thieves, in two cars, surrounded the van and forced it off the A6 road between Paris and Lyon on Monday, a spokesman for the national gendarmerie said.
They then seized the gold, locked the two security workers in the back of the van and set one of their own cars on fire before fleeing, according to the spokesman.
He said the flames spread to three other cars and threatened to engulf the van before police intervened to rescue the two men inside after being tipped off by a witness. The other cars affected appeared to have been parked and unoccupied, he said.
Police vehicles and a helicopter were searching the area around the town of Dardilly, the spokesman said, speaking on condition of anonymity because he was not authorised to give details about an ongoing investigation.
Forensic police were examining the burnt-out cars and the empty van, operated by the security company Loomis.
Big jewel thefts occasionally occur in France, including some high-profile highway heists, but it was rare to see a large-scale gold robbery such as this, gendarmes said.
The spokesman said dozens of kilograms were stolen. With gold selling for more than $37,000 (£30,000) a kg, the value of the haul could be more than £700,000.
Authorities did not identify the suspected attackers or release details of who owned the gold or where it was being taken.