Jewellery worth £500,000 was stolen in an “audacious” raid on a shop in an historic part of central London.
The burglars who struck at George Attenborough and Son Jewellers on Fleet Street at the weekend smashed through a wall to get into the store.
Thousands of runners were taking part in a half marathon nearby and the noise may have masked what they were doing.
It is also believed that they painted over a CCTV camera in a bookmakers next to the jewellers to cover their tracks.
Detective Superintendent Simon Moring said: “This was an audacious burglary, and there are elements which appear to have been planned with some precision by the suspects.
“They broke through a wall in order to gain access to the store.
“At this stage we are not releasing any further details about the methods used by the suspects.
“It is nowhere near the sophistication of some previous burglaries dealt with by the Flying Squad, including high profile crimes such as the Hatton Garden burglary in 2015.
“The estimated loss is £500,000.
“This is a theft of high value which warrants thorough and determined investigation.”
He appealed to anyone who was in the area on Saturday and Sunday to consider if they saw something that was “out of place”.
The Sun reported that the gang had used drills to smash their way through a wall from a neighbouring premises.
Another theory was that the burglars got into the main building through a fire escape, opening a door by reaching through a letterbox.
A camera in a branch of William Hill, next door to the jewellers, in central London, was found to have been daubed with white paint after the raid at the weekend.
Speaking at the scene on Wednesday, former Metropolitan Police detective Peter Kirkham told reporters: “Although this is not one of those things where only a couple of gangs could be capable of pulling it off, it would be above the average amateur burglar, some thought has gone into it.
“There’s probably some sort of alarm system they’ve had to deal with, so there was a degree of difficulty.”
Speaking of the evidence available to detectives, Mr Kirkham said: “It’s very difficult not to leave a trace of yourself these days, fingerprints are easier to avoid leaving but DNA is a totally different matter, so they may well have left some traces there.
“CCTV will play a big part, police can check every shop up and down Fleet Street, they’ll be able to check for any comings and goings, if they were in there for a length of time and it’s quiet, which it usually is on Saturday.”
Detectives from the Metropolitan Police anti-robbery Flying Squad are leading the hunt for the burglars.
Anyone with information can call police on 101 quoting CAD 451/March 25, or to remain anonymous Crimestoppers on 0800 555 111 or online.
The incident happened a short distance from the Hatton Garden Safe Deposit company, which was raided in April 2015.
Of the £13.6 million of property stolen in the heist, only around £4.5 million – roughly one-third – has since been recovered by police.
Earlier this month, alarm specialist Michael Seed – known by the moniker Basil – became the 10th person convicted in connection with the crime when he was found guilty of conspiracy to burgle and conspiracy to handle the proceeds after £143,000 worth of gold ingots, gems and jewellery was found in his bedroom.