Southern Monitoring Alarm Company called the Metropolitan Police Service, also known as Scotland Yard, at 12:21 a.m. April 3 to report that the burglar alarm had been activated at Hatton Garden Safe Deposit Ltd., MPS said in a prepared statement.
"The call was recorded and transferred to the police's CAD (computer-aided dispatch) system," the statement said. "A grade was applied to the call that meant that no police response was deemed to be required. We are now investigating why this grade was applied to the call. This investigation is being carried out locally.
"It is too early to say if the handling of the call would have had an impact on the outcome of the incident."
The theft was so big that police haven't come up with a value for what was stolen.
Over the four-day Easter holiday, thieves broke into the vault of Hatton Garden Safe Deposit Ltd. and might have been able to take as long as four days to rifle through the boxes.
A former police official in London has speculated that the loss could run to £200 million, or $300 million, in a remark widely reported by news media. Numerous British news organizations put the value of the loss in the hundreds of thousands of pounds.
Detective Chief Inspector Paul Johnson of the London Metropolitan Police Flying Squad said police were still identifying the owners of ransacked safe deposit boxes and trying to contact them to learn what had been lost.
Tabloid says it has closed circuit TV footage
The British tabloid The Daily Mirror claimed Friday to have obtained closed-circuit TV footage that captured the robbery being carried out. The video showed people inside the building dressed like utility workers with their faces covered. They carried large bags, what looked like drill equipment and other tools, then exited with trash bins.
Toward the end of the video, a white van can be seen on a street during daytime with individuals loading back their gear and the trash bins.
British police told CNN they have not released any video of the heist. When asked about the video published by the Daily Mirror, police said they could not confirm that it was footage from the Hatton Garden robbery and that officers have not seen that particular video.
Images said to show gang's comings and goings
The Daily Mirror published time-stamped images it said showed that the thieves had been -- as was feared -- in the vault for days.
The Mirror's time stamps, which CNN has not been able to independently verify, show employees locking up for the weekend at 9:19 p.m. on Thursday. If the footage, and its interpretation by the newspaper are correct, at least six people were involved in the heist
Just four minutes later, the first of the thieves, nicknamed "Mr. Ginger" by the newspaper for his red hair, appears in the building holding a black bag. He goes downstairs toward the vault.
At 9:27 p.m., a street camera shows a white Ford Transit van pulling up to an alley beside the Hatton Garden Safe Deposit building. Men are seen dragging trash bins down the alley before the van drives away, leaving the men behind.
Inside the building, at 9:30p.m., the camera records a thief nicknamed "The Gent" for his natty clothing -- though he also wears a hard-hat and a high-visibility jacket labeled "Gas" on the back.
A minute later, "Mr. Strong" appears, wearing a builder's hat and carrying steel supports, which the newspaper speculates could have been used to support the diamond-tipped drill used to smash through the wall of the vault.
At 9:36 p.m., Mr. Strong and a thief nicknamed Mr. Montana for the logo on his hooded sweatshirt roll in trash bins, one of which might have contained, according the Mirror, the 77-pound drill that bored through the reinforced seven-foot concrete wall to reach the vault.
It is unclear, but the thieves may have spent the night in the basement in or near the vault. At any rate, according to the Daily Mirror, no more activity is seen above ground until Friday morning, shortly before 8:00.
The white van returns, is loaded in two minutes, and drives off again.
On Saturday evening, Mr. Ginger returns, two days after he was first seen. The newspaper says is wearing latex gloves and carrying a black sack. He goes downstairs toward the vault.
Saturday evening also marked the first appearance of the Tall Man, who helped carry some of the loot out of the building.
Early Sunday, Mr Ginger, the Tall Man and a robber nicknamed the Old Man are seen to be active. The Tall Man and the Old Man struggle to move a bin before they drag it outside. The Old Man leans on the bin, struggling for breath, and reveals the side of his face to the camera.
A white van arrives by the alley and the men start loading equipment on it, including several trash bins. Three men get into the white van and, at 6:44 a.m. they are gone.
Heist not reported until Tuesday morning
The heist would not be reported to police for two more days, on Tuesday morning when employees of the company arrived for work.
Police said Thursday there was no sign of forced entry. Johnson said the thieves appeared to have gained access to the vault through the shaft of an elevator that is used by several businesses in the building.
The thieves disabled the elevator on the second floor of the building -- which would be called the third floor in the United States -- then climbed down the elevator shaft into the basement, he said.
Once there, he said, they used a drill to bore through a 6-foot-thick wall and gain access to the vault where the safe deposit boxes were.
Cash, jewels probably taken
People with knowledge of the area have speculated that cash and jewels were probably taken. Some jewelry businesses reportedly stored some of their jewels in the boxes rather than leaving it in their stores over the holiday weekend.
Johnson said the scene in the vault remained chaotic as police continued their forensic examination. He said the floor was covered with dust and littered with safe deposit boxes and power tools.
Johnson called the crime sophisticated and said there were a limited number of people in the United Kingdom capable of pulling it off. He said he had no idea whether the thieves were still in the country.
Although there was no sign of forced entry to the building, the detective said, "whether that involves inside knowledge will form part of the investigation."
Hatton Garden is a storied area in London and the heart of the city's diamond trade. The area's promotional website
says it is home to "the largest and most concentrated cluster of jewellery retailers in the UK" and has been for quite some time.
"History tells us that the old City of London had certain streets -- or quarters -- dedicated to specific types of business," the website says. "The Hatton Garden area has been the epicentre of London's jewellery trade since medieval times.
"Today, it maintains its international reputation as the centre of London's diamond trade. It is one of the finest and most renowned jewellery locations in the world."
The website of Hatton Garden Safe Deposit Ltd. says the company was founded in 1954 and offers a "secure and cost-effective solution to store and protect important and irreplaceable personal belongings."