The FBI has been locked out of almost 7,000 seized mobile phones thanks to encryption, director Christopher Wray has said.

Speaking at the International Association of Chiefs of Police conference in Philadelphia in the US, Wray lamented that device encryption kept the g-men out of “more than 6,900, that’s six thousand nine hundred, mobile devices … even though we had the legal authority to do so.”

"It impacts investigations across the board: narcotics, human trafficking, counterterrorism, counterintelligence, gangs, organized crime, child exploitation," he added.

Device encryption, where a mobile phone encrypts information stored on it, is a useful security measure in cases where phones are stolen. However, if a device owner arrested by police refuses to decrypt it, cops would have to crack the device to read messages stored on it.

The problem does not arise in the UK, where it is a criminal offence to refuse to give your password to State investigators.

Wray later added in his speech: “I get it, there's a balance that needs to be struck between encryption and the importance of giving us the tools we need to keep the public safe,” according to the BBC.

Device encryption is a separate thing from end-to-end encryption, which protects messages and phonecalls from being intercepted and read over the air as they travel between the device and the server. Amber Rudd, the Home Secretary, has repeatedly attacked technology companies for their use of end-to-end encryption, which makes it far harder for small State agencies to use their Investigatory Powers Act powers to spy on alleged miscreants.

In 2014 British police complained that another phone security measure, the ability to remotely wipe the device, was being used to cleanse phones seized in criminal investigations before police could read them. ®