The devices are installed in places like train stations or public buildings where they scan passers by to see if they are acting suspiciously. Using a range of in-built parameters of what is ‘normal’ the cameras then send a text message to a human guard to issue an alert – or call them.

Manufacturers BRS Labs said it has installed the cameras at tourist attractions, government buildings and military bases in the U.S. But the Texas-based company has offices in London, Sao Paulo, and Barcelona – meaning they could be in dozens of places around the world.

In its latest project BRS Labs is to install its devices on the transport system in San Francisco, which includes buses, trams and subways.

The company says will put them in 12 stations with up to 22 cameras in each, bringing the total number to 288. The cameras will be able to track up to 150 people at a time in real time and will gradually build up a ‘memory’ of suspicious behaviour to work out what is suspicious.

BRS Labs said the cameras effective have ‘the capability to learn from what they observe’.