Moscow is adding facial-recognition technology to its network of 170,000 surveillance cameras across the city in a move to identify criminals and boost security.

Since 2012, CCTV recordings have been held for five days after they’re captured, with about 20 million hours of video stored at any one time.

“We soon found it impossible to process such volumes of data by police officers alone,” said Artem Ermolaev, head of the department of information technology in Moscow. “We needed an artificial intelligence to help find what we are looking for.”

Moscow says the city’s centralized surveillance network is the world’s largest of its kind.

The United Kingdom is one of the most notorious for its use of CCTV cameras but precise figures are difficult to obtain. However, a 2013 report by the British Security Industry Association estimated there were as many as 70,000 cameras operated by the government across the nation.

Moscow’s facial-recognition technology was designed by Russian startup N-Tech.Lab Ltd. The system cross-references a digital fingerprint of images from the Interior Ministry’s database against those captured by cameras at entrances to apartment buildings, Ermolaev said. A two-month trial of the system earlier this year resulted in six criminals being detained from a federal “wanted” list, he said.

Ntechlab’s accuracy has been recognized by the U.S. Department of Commerce and the University of Washington. The Moscow-based company released a mobile app called FindFace last year that became a big hit in Russia.

Consumers would take a photograph of strangers in public spaces and the tool would identify those in the picture by matching their faces to profiles on Russia’s largest social network, VKontakte, with a reported 70 percent accuracy rate.

Facial recognition technology has been creeping into the consumer market for a number of years, with increasing accuracy and acceptance by users. Microsoft’s Windows 10 operating system can be unlocked when an authorized user sits in front of a computer’s webcam and Samsung Electronics includes a similar technology in its smartphones. More recently, Apple announced its flagship iPhone X would dispense with a fingerprint security sensor in favor of facial recognition.

Ermolaev said the Moscow government is already spending about $86 million a year to maintain its video surveillance system. Deploying facial-recognition to all of 170,000 cameras would have tripled these costs, he said, so the technology will be applied selectively within districts where it’s currently most needed.

Such systems are legal in Russia, says Mikhail Zyuzin, an IT expert at the Moscow-based Academy of Information Systems. It causes concerns about personal privacy, however.