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, an unknown number of 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.

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 a building dressed like utility workers with their faces covered. They carried large bags, what looks 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.

CNN has not independently verified the authenticity of the video and 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.

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.

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.