The news came after detectives said they had recovered guns, clothing and about 1.3 million pounds in cash from an abandoned white van found on Saturday not far from the scene of Wednesday's robbery in the town of Tonbridge.
They also found body armour which they believe was used in the cash heist, thought to have been the biggest in the country's history.
Assistant Chief Constable, Adrian Leppard, said a 49-year-old man had been arrested on Saturday in connection with the van and had later been released on police bail.
Two other men, aged 33 and 55, arrested in the Maidstone area on suspicion of conspiracy to commit robbery on Saturday, have also been released on bail.
"During the last 24 hours we have executed a number of search warrants. A number of those search warrants because of the risk posed have involved pre-planned firearms operations," Leppard told a news conference.
He said he could give no details about the searches or whether they had resulted in further arrests.
Robbers are said to have used a
postal van for their abduction
"I'm sure you'll understand the sensitive nature of the investigation at this stage," he added.
Sky News reported that two people had been arrested in the Tankerton area of Kent after police stopped their car by shooting at the tyres.
However, police said they could not confirm the reports.
A massive forensic investigation is now on to study the items found in the van and to examine other vehicles which police have seized and believe were used by the six robbers.
They are also studying 14 steel cages which they believe were used to move the stolen cash.
Police issued a handout sketch
of a suspect
Leppard said police had received more than 1000 calls to their hotline number.
He said he believed that the mounting pressure from the massive police investigation was starting to panic members of the gang into making mistakes.
"They are 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," Leppard said.
Six robbers, posing as police officers, kidnapped Colin Dixon the manager of the security depot in Tonbridge, his wife Lynn and young son Craig to carry out the massive raid.
Depot manager Dixon issued a statement on Saturday describing what he and his family had endured as the worst night of their lives.
He said his son Craig, who was nine on Friday, was still in deep shock.
"The terror of what happened and the horror of what might have happened is with us in every waking moment," Dixon added.
"For the criminals to use me is bad enough, but to kidnap my wife and child and put guns to their heads and threaten them with death, is something so frightening that no words can convey them.
"I mean, how would you have felt? All that is precious to you - your family - placed in unimaginable danger through no fault of their own, entirely because of someone else's greed."