Hackers from the Anonymous group hijacked the website of the United States Sentencing Commission, which is responsible for federal sentencing.
The group said it did so to avenge the death of Aaron Swartz, an computer prodigy who died on 11 January.
Mr Swartz, 26, was facing hacking charges in the United States. The BBC reports he is believed to have taken his own life.
His trial was due to be held next month. If found guilty, he could have faced up to 35 years in prison.
Before the website was taken down, Anonymous posted a message on Saturday saying it had infiltrated several government computer systems and copied secret information, which it threatened to make public throughout the day.
The FBI is investigating the incident.
"We .. are handling it as a criminal investigation," said Richard McFeely of the bureau's criminal, cyber, response, and services branch in an emailed statement.
The BBC reports there have been numerous attacks by hackers operating under the Anonymous banner recently. Government websites in Britain and other countries have been the targets.