A group that says it targets online activity linked to so-called Islamic State (IS) has claimed it was behind an attack on the BBC's website.
All the BBC's websites were unavailable for several hours on New Year's Eve after what a BBC source described as a "distributed denial of service" attack.
The group, calling itself New World Hacking, said it had carried out the attack as a "test of its capabilities".
The BBC has not confirmed or denied such an attack caused the problems.
The corporation's press office said on Saturday that the BBC would not be commenting on the group's claim.
A "distributed denial of service" attack, which the group claims it carried out, aims to knock a site offline by swamping it with more traffic than it can handle.
In a tweet to BBC technology correspondent Rory Cellan-Jones, the group said: "We are based in the US, but we strive to take down Isis [IS] affiliated websites, also Isis members.
"We realise sometimes what we do is not always the right choice, but without cyber hackers... who is there to fight off online terrorists?
"The reason we really targeted [the] BBC is because we wanted to see our actual server power."
Earlier, New World Hacking had said: "It was only a test, we didn't exactly plan to take it down for multiple hours. Our servers are quite strong."
One of the group's members - nicknamed Ownz - told the BBC News website's Leo Kelion that New World Hacking was a team of 12 people - eight male and four female - who came together in 2012.
The group's other recent activities included taking part in a campaign against the Ku Klux Klan, and the #OpParis effort to identify and report IS social media accounts following the November attacks on the French capital, Ownz told the BBC.
Ownz said his group used a tool called Bangstresser - created by another US-based "hacktivist" - to direct a flood of traffic against the BBC, and had supplemented the attack with requests from its own personal computer servers.
The group has already used the technique against IS websites, but intended to "really get into the action" against a new list of targets associated with the militant Islamist group from Tuesday, Ownz claimed.
The problems on the BBC sites began at about 07:00 GMT on Thursday, and meant visitors saw an error message instead of the intended content.
The attack hit the main BBC website as well as associated services including the iPlayer catch-up service and iPlayer Radio app.
An initial statement tweeted by the BBC blamed the problems on a "technical issue". The corporation said it was working to make sites, services and pages reachable again.
By 10:30 GMT the site was largely working again although some pages and indexes took longer than normal to load.
At midday on Thursday, the BBC said its websites were now "operating normally", and apologised for any inconvenience caused.