The Metropolitan police said investigations were continuing into the decision not to send a police officer to investigate the alarm, which was triggered after the robbers had been inside the building for three hours.
The new detail about the heist came as police, believed to have not yet identified prime suspects, released new pictures showing the scene of the robbery in Hatton Garden.
Part of the efforts to work out how the robbery took place involves trying to establish how easy it would be to climb through the small hole drilled through the concrete wall of the vault at Hatton Garden Safe Deposit. Police said the hole was 50cm deep, 25cm high and 45cm wide. It was 89cm off the ground and police were surprised by the tiny size of the hole bored through the concrete wall, bypassing the vault’s reinforced metal doors.
A diamond-tipped drill bit on a Hilti DD350 was used and police say the gang left behind various tools, including drills, as well as crowbars and angle-grinders probably used to force the boxes open.
According to police, the gang drilled through a thick concrete wall before rifling through 72 secure boxes believed to contain cash, jewels and other valuables. Officers are trying to find out why only 72 of 999 boxes were opened.
The images show the scene officers found when they arrived after the raid over the Easter weekend: the floor was covered in debris and strewn with discarded deposit boxes and power tools.
After studying CCTV footage and clues, detectives believe between four and six people broke into the vault. The Met said they first entered at about 9.20pm on 2 April and stayed until 8.05am the next morning, Good Friday. An alarm went off at 12.21am on 3 April, about three hours after the gang entered the vault area, according to timings released by police.
The gang returned to the vault on Saturday 4 April at about 10.17pm, staying until 6.30am the next morning. Key staff were off work because of the Easter holiday and police were alerted to the burglary just after 8am on Tuesday 7 April.
When they arrived, detectives found no sign of forced entry outside the building. A communal lift on the second floor had been disabled and the suspects had used the lift shaft to climb down into the basement. There, they had forced open shutter doors and drilled holes into the wall of the vault.
Police say they have recovered thousands of hours of CCTV footage and are gathering all the clues left behind in the hope of catching the robbers. DS Craig Turner, head of the Flying Squad, said: “The hours of forensic work and inquiries have been vital in order to ensure we are able to exploit all investigative opportunities to their fullest extent and assist us in identifying those individuals responsible.
“Those safety deposit boxes not opened by the thieves during the burglary have been left secured as they were found throughout the examination. HGSD are in the process of making contact with owners to arrange collection.”
The Met said: “The team have recorded, packaged and recovered approximately 400 exhibits, including items for DNA profiling, fingerprints and other evidence.
Specialist forensic photographers have mapped out the crime scene and utilised digital techniques to record the inside of the premises.”
The Met said it was reviewing why no officer was sent after the alarm went off at the premises. “It is too early to say if the handling of the call would have had an impact on the outcome of the incident,” Scotland Yard said in a statement.
“An alarm call from Southern Monitoring Alarm Company was received at the Met’s central communications command at 12.21am on 3 April, saying that an intruder alarm had been activated at the Hatton Garden Safe Deposit Ltd.
The call was recorded and transferred to the police’s computer-aided dispatch system. “A grade was applied to the call that meant that no police response was deemed to be required,” the statement said. “An internal investigation is ongoing to identify why this grade was applied to the call in conjunction with the alarm company.”
Millions in cash and valuables were stolen during the raid. Those who had rented boxes at the safe deposit company have expressed anger at the burglary and police failure to respond to the alarm.
No arrests have been made and detectives are not yet believed to have identified prime suspects. An appeal for information about the heist and its perpetrators will be made on BBC Crimewatch on Thursday eveningHatton Garden jewellery heist: police appeal to partners of drill experts