A £2m reward has been offered after armed robbers stole up to £50m in a raid on a Securitas depot in Kent.
Police have said the raid was carried out with military precision by professional robbers operating at the top level of organised crime.
The security firm and insurers put up the reward to try to tempt criminals with information to come forward.
Kent Police said they had never offered such a large reward, thought to be the biggest in the UK.
Kent Assistant Chief Constable Adrian Leppard said many people would have planned the crime over weeks or months.
"This is organised crime at its top level. This was planned and executed with military precision," he said.
"This is a terrifying crime committed by professional and organised gangsters."
Colin and Lynn Dixon were taken to the depot with their son
The Crimestoppers Trust said it believed the £2m offered for information was the largest in UK history, although huge bounties were regularly offered in the USA for the capture of high-profile criminals.
Mr Leppard said: "This is a significant reward, which reflects the serious nature of this robbery.
"It is important to remember that this is a dangerous armed gang, which put a young family, including an eight-year-old boy, and 14 staff at the depot through a terrifying ordeal."
Mr Leppard said it was not known whether inside knowledge had been involved or whether the operation had been planned using outside reconnaissance work.
Officers were keeping a open mind about whether the gang would have tried to leave the country, but the borders were being checked, he added.
The assistant chief constable said at least £20m cash was missing, but the amount taken could be as high as £50m.
The precise total will not be known until auditors have been allowed into the site.
He said detectives were liaising with the National Crime Squad, the National Criminal Intelligence Service, and had spoken to police in Northern Ireland about previous robberies.
Police said the gang of at least six men, some wearing masks and carrying handguns, were "armed, dangerous, and violently threatening".
They first struck on Tuesday evening, when manager Colin Dixon, 51, was abducted by men posing as police officers.
The 7.5 tonne lorry used by the robbers was caught on CCTV
Two further gang members, again posing as police, visited the manager's Herne Bay home telling his wife Lynn, 45, and son, Craig, he had been involved in an accident, before taking them to a west Kent farm where the manager was held.
The family was then taken to the Vale Road depot where the manager was forced to let one of the gang in.
Fourteen staff members were tied up and kept in a room.
The money was loaded on to a white Renault lorry before the gang drove off early on Wednesday.
Securitas' security director, Paul Fullicks, said: "Managing people in a security environment is always very difficult. We continually review security to try to eliminate weaknesses.
"In this instance, people have been the weak link. Until we can speak to them, we won't know if anything went wrong."
Anyone with any information is asked to contact police incident rooms on 01622 652361 or 01622 652366.
1) Security depot manager abducted by gang members posing as policemen, close to the Three Squirrels pub, near Stockbury
2) The manager's wife and son, aged eight, are taken from their home in Herne Bay, after being told he was in an accident
3)Wife and son taken to farmhouse at unknown location in west Kent. The manager is eventually taken there as well
4) Armed and masked robbers raid the depot in Tonbridge