The UK Serious Organised Crime agency has taken its website offline after it appeared to be a victim of an attack by hacking group Lulz Security.
Soca said it had taken its website offline to limit the impact attack on clients hosted by its service provider.
Soca.gov.uk had been unavailable for much of Monday afternoon, with an intermittent service restored later.
Lulz Security has said it was behind the denial of service attack which had taken the website offline.
Earlier on Monday, as the agency launched an investigation, LulzSec tweeted: "Tango down - in the name of #AntiSec".
The group has hit a number of high-profile websites in recent weeks, including the CIA and US Senate.
Soca appeared to be the victim of a distributed denial of service (DDoS) attack, where large numbers of computers, under malicious control, overload their target with web requests.
In a statement given to BBC News, a Soca spokesman said: "Soca has chosen to take its website offline to limit the impact of DDoS attack on other clients hosted by our service provider.
"The Soca website is a source of information for the general public which is hosted by an external provider. It is not linked to our operational material or the data we hold."