Hatton Garden raid: Police failed to respond to alarm
Police were told an intruder alarm had gone off at the scene of the Hatton Garden safe deposit box raid but decided it did not require a response.
About 70 safety deposit boxes were opened at the Hatton Garden Safe Deposit Ltd in central London over the Easter Bank Holiday weekend.
The Met Police said they received a call on Friday 3 April at 00:21 BST.
Officers are now investigating why the call was given a grade that meant no police response was deemed necessary.
In a statement, the Met Police said: "It is too early to say if the handling of the call would have had an impact on the outcome of the incident."
Meanwhile CCTV footage has emerged which appears to show the group of burglars behind the raid.
The video, obtained by the Mirror, shows men in high visibility jackets outside the safe deposit building.
One is seen pushing a wheelie bin, which the paper said could have contained the large drill used for breaking a hole through the lift shaft.
The Mirror said the men made two separate visits before leaving on Easter Sunday with the contents of 72 safety deposit boxes in wheelie bins and bags and making a getaway in a white van.
A source told the paper the raid was the work of a professional gang who "planned this job down to every last detail".
Scotland Yard has declined to comment on the footage.
The Met Police has said items were stolen from at least 56 safety deposit boxes and officers are in the process of contacting the holders.
In total, 72 boxes were opened. Five were vacant and 11 were due to be opened by the company following the non-payment of fees, police said.
BBC Home Affairs Correspondent Danny Shaw said: "It is quite possible - and this is certainly one aspect that the inquiry will be looking at - that there had been so many false alarms previously that the police had decided not to respond to any more alerts.
"There is a police policy that if there are three false alarms in a 12-month period they won't respond anymore when intruder alarms go off and it is possible that may well have been what happened here."
He added that the security guard at the building had also been spoken to by officers.
The force said the investigation into why the grade was applied to the phone call would be carried out locally.
Southern Monitoring Alarm Company contacted the Metropolitan Police central communications command and the call was recorded and transferred to the computer aided despatch system.
The call stated a confirmed intruder alarm had been activated at the Hatton Garden Safe Deposit Ltd, police said.
According to the Met's website, calls to the communications command are dealt with by an operator, who grades all incidents "in terms of their urgency".
The website states: "First contact operators will question the caller and gain all the relevant information necessary to ensure the best police response.
"Having completed this, the operator will grade the call in accordance with standard operating procedures for the type of incident."
Calls are passed to a despatch operator for deployment if required and police resources are assigned depending on the type of incident, the site says.
Previously, Scotland Yard said they were alerted to the burglary on Tuesday.
The Met said it was "photographing the scene and recovering exhibits in meticulous detail". It would not comment on the total value of items stolen.
A heavy duty drill was used to bore holes into the 6ft (2m) thick walls reinforced with concrete to access the vault after the thieves had climbed down a lift shaft.
Experts said it was likely the thieves made several holes until they had created a big enough space to get through, which would have taken an experienced operator several hours.
Hatton Garden is famous for jewellery and the vault was used mainly by local jewellers and gold dealers.
No arrests have been made.