Guns, balaclavas and flak jackets along with cash were all recovered from a van by investigators of the Ј50m Securitas robbery in Tonbridge, Kent.
Detectives said leaving the items in the Ford Transit shows the gang are "making mistakes" and feel "pressured".
Assistant Chief Constable Adrian Leppard, of Kent Police, said "the net is closing in" on the raiders.
On Saturday two men, aged 55 and 33, were arrested in Maidstone on suspicion of conspiracy to commit robbery.
Police also said a red Vauxhall Vectra, found burnt out, could have been used in the raid last Wednesday.
The Vectra was next to a Volvo saloon, in Leeds, near Maidstone, that was also burnt out and might have been used to abduct depot manager Colin Dixon in the first stage of the heist.
Asst Ch Con Leppard confirmed the items found in the van, in the car park of the Ashford International Hotel, Ashford, included jackets that are "similar in style to body armour".
"This discovery is important because of the descriptions we have of the robbers," he said.
"Leaving firearms, clothing and cash in the van suggests to me that there are people in the gang who are now making mistakes.
"They're under pressure and we want to keep the pressure on. I am very encouraged at this stage by the progress we are making. The net is closing in."
A computer image of another raider has been released.
The image of one of the two men who abducted Lynn Dixon and her son, Craig, shows a man wearing dark-style police clothing and possibly a wig and a false moustache.
He is described as white, skinny, 6ft to 6ft 2in, and in his mid to late 40s. He had a pasty complexion and a long thin nose, thin lips, a bony chin, thick, brown unkempt hair and a bushy dark brown moustache.
It follows the release of another image of one of the men who kidnapped Mr Dixon.
Tip-off
On Saturday, police read out a statement from Mr Dixon describing the terror he and his family went through.
"The horror of what might have happened is with us in every waking moment," he said.
On Friday, a tip-off from a member of the public led to the white Transit van being found in Ashford with a large amount of money in it, thought to be from the raid.
Earlier, police revealed that several vehicles used in the raid had been found.
Three people arrested during the investigation have been released on bail.
(BBC)
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