Obscene image shown on hacked US billboard Published duration 21 May 2015

image copyright Thinkstock image caption An easy-to-guess password is believed to have let attackers swap the image

Hackers have managed to make a huge video billboard in Atlanta display an obscene image favoured by internet pranksters.

It prompted calls to police, and soon after, the billboard's owner cut off its power supply.

The hack came after a security researcher warned the company, which runs thousands of the video billboards, that they were vulnerable to attack.

The FBI and Homeland Security are now investigating the hack.

The attackers are believed to have been able to take over the billboard because it used an easy-to-guess password on its net-connected remote administration system.

The billboard is owned by US electric-sign giant Yesco, which runs thousands of similar billboards across America.

Other signs in other states are also believed to have had their images changed at the same time.

Security expert Dan Tentler said in a tweet that he had warned Yesco about the potential for attack, but the company had told him that it was "not interested" in his information.

Mr Tentler said it was easy to find hundreds of other signs on the internet that were vulnerable in the same way.

Many of these signs were still online after the hack attack had taken place, he added.

A group calling itself the Assange Shuffle Collective claimed responsibility for the attack, in a discussion on social news site Reddit.

However, there has been no independent verification of the claim.