The van used to carry off the £26.5m stolen in a raid on the Northern Bank in Belfast crossed the Irish border hours before the robbery, police said.
The van, bottom left, is seen close to the bank
Fresh information about the raid and the haul of notes the thieves escaped with has been released by detectives.
Police appealed to the couple who reported suspicious activity near the Northern Bank on the night of the raid to come forward.
The couple alerted a traffic warden to suspicious activity close to the bank.
Trace the couple
It has now emerged that they told a traffic warden they had seen two men apparently wearing wigs and carrying baseball bats at the van.
A foot patrol was sent but arrived about three minutes after the gang had left.
Police are keen to trace the couple and also the van used to carry off the money.
They said they were aware that the van crossed the border on the main Dublin to Belfast road about 1700 GMT on 20 December, the day of the robbery.
It was assumed by police that it took the main road into Belfast, where its movements have been tracked on CCTV footage, twice going to the back of the bank
in Donegall Square to load up with cash.
Northern Bank is to withdraw its bank notes
Detective Superintendent Andy Sproule said: "We don't know where the van went after it left the
scene. Somebody must have seen it. People would know what happened. Please tell us."
It also emerged that the thieves have about £10m in notes for which the authorities have no serial numbers.
Police said £4m was in £20 notes from other banks, much of it untraceable.
The rest is in Northern bank used £20s and about £1.15m in new Northern Bank £100 and £50 notes for which there are no serial numbers.
Northern Bank last week announced plans to withdraw its entire issue of notes, with the exception of £5 notes that were not involved in the robbery, and
replace them with a new issue.
However, that is expected to take several weeks
Mr Sproule also revealed that at the moment the Northern Bank had decided
against offering a reward.
He said his officers were engaged in a major investigation and had more than
800 lines of inquiry.
Some 140 statements had been recorded and 700 exhibits
He said it would be a long and time-consuming investigation.
"This is the real world, not like TV where crimes are solved in 40 minutes.
"We are continuing to work to identify and bring to court those who carried out this robbery, but it will be a long investigation during which no stone will be