Hackers have targeted the website of the National Crime Agency (NCA), the department set up to tackle organised crime, in an apparent revenge attack.
A Twitter account linked to the hacking group Lizard Squad appears to have claimed responsibility.
The NCA's site was inaccessible for some time on Tuesday morning.
The attack comes days after six teenagers were arrested by the NCA for using a service known as Lizard Stresser.
The 15 to 18-year-olds are believed to have targeted organisations including a national newspaper, a school, gaming companies and some online retailers.
It appears the NCA website was taken down in a distributed denial of service (DDoS) attack, where websites or servers are flooded with requests for data.
A spokesman for the National Crime Agency said its website was an "attractive target" and attacks are "a fact of life".
He said: "DDoS is a blunt form of attack which takes volume and not skill.
"It isn't a security breach, and it doesn't affect our operational capability.
"At worst, it is a temporary inconvenience to users of our website."
XBox and PlayStation attacks
Lizard Stresser is a tool which allows people to pay a fee to nominate a website they would like to be targeted by a DDoS attack.
It became popular after Lizard Squad took responsibility for several high-profile attacks.
The group claimed to have taken XBox and PlayStation gaming networks offline last Christmas.
None of the teenagers in these recent arrests are accused of involvement in those attacks or believed to be members of Lizard Squad.
The National Crime Agency said they are suspected of deploying Lizard Stresser, having bought it using services such as Bitcoin in an attempt to stay anonymous.
The targets haven't been named and it hasn't been confirmed whether the attempted DDoS attacks were successful.
As part of the same operation, officers are visiting around 50 addresses linked to people who are registered on the Lizard Stresser website - but who are not suspected of involvement in attacks.