The Federal Bureau of Investigation said steps were being taken to "further insulate" its site.

Hackers have forced the closure of the FBI and the Senate sites.

The Senate site was closed after being defaced with an obscene message, which also poked fun at the FBI by asking: "Who laughs last?"

Hackers who left the message said the attack was partly in response to what it called the FBI's harassment of specific hacker groups.

These include one group that claimed responsibility for breaking into the White House site earlier this month.

The FBI site has been closed since Wednesday and there are no indications when it may be online again.

An FBI statement said: "There was an attempt by unknown persons to unlawfully gain access to the FBI Website. It was unsuccessful.

"However, as a precaution, the FBI shut down the site and is now taking additional steps to further insulate it."

Hacker investigations

FBI spokesman Rolando Moss said agents have been investigating allegations of computer intrusions involving a Houston hacker.

And earlier this month, a grand jury in Virginia indicted Eric Burns, 19, on three counts of computer intrusion.

Mr Burns is reportedly known on the Internet as 'Zyklon'. He is believed to be a member of the group that claimed responsibility for an attack on the White House.

The White House site was altered for a few minutes on Sunday before government computers automatically detected the intrusion.