Two dark-skinned men in their 40s robbed Samco staff - Rachan Janratsami and Pongsak Khamwong - at gunpoint at about noon as they transported the cash down a side alley by the bank branch. The two guards were to due to deliver the money to Krungsri Ayudhya Bank head office on Ploenchit Road.

The gunmen demanded the cash bag and forced the pair to lie face down, before they escaped in a bronze Vigo pickup along Srinakarin Road towards Klong Ton. The pickup had the registration number Por Ngor 6815 Bangkok. It was later found to be fake.

Police set up checkpoints at possible escape routes, said Metropolitan Police deputy commander, Major General Kridsada Phankhongcheun. He said the security van had earlier collected money from seven branches and police believed there were at least three people involved.

Kridsada said police were also investigating whether there was an insider involved, and would check records of the Samco guards' cell-phones prior to the heist.

Police later yesterday collected fingerprints from the security van for possible leads, while Samco sent another truck to deliver money to the headquarters as scheduled.

Deputy Metropolitan Police chief Maj-General Jes Mongkolhatthi also asked for the bank's security camera footage to see if the thieves had scouted the location prior to the robbery. He raised the question of why the security van, which usually parked in front of the bank to collect money, on this occasion parked in the side alley.

Rachan said he put the cash bag on the ground to unlock the van door - as the van required two guards to turn their keys at the same time - when two gunmen pointed guns at the back of their heads and made them lie face down.

Rachan said he was so scared he did not dare look up, but caught a glimpse of one robber, who had curly hair and a strange voice.

After the thieves escaped, Rachan said he called for help and promptly reported the incident to his company. He also insisted that, each day, he and his colleague did not know in advance what routes would be used to collect money.

A source said police believed the robbers might be members of a "Vigo Gang", which has robbed gas stations, 7-Elevens and banks in Bangkok and nearby areas.