The hacker activist group Anonymous hacked into a Justice Department server and posted troves of data online on Monday.
The group claimed it was releasing 1.7GB of data belonging to the "United States Bureau of Justice" and said the data included internal emails.
But she said the only server affected was the one that contains the data for the public website of the Department's Bureau of Justice Statistics, which compiles national crime statistics.
The spokeswoman said the Bureau of Justice Statistics website had remained operational and the main website, justice.gov, was not affected.
"The department is continuing protection and defensive measures to safeguard information and will refer any activity that is determined to be criminal in nature to law enforcement for investigation," she said.
It is not immediately clear whether the data dump included any sensitive information. But the fact that Anonymous had gained access to a Justice Department server makes the attack much more serious than the group's more common tactic of overloading a government website with requests from thousands of computers.
Anonymous has used that method, known as a denial-of-service attack, in the past to crash the websites of the Justice Department, the FBI, the CIA and major record labels and movie studios. But that method does not give the hackers access to classified information.
In a statement, the hacker group said it released the data "to spread information, to allow the people to be heard and to know the corruption in their government."
"We are releasing it to end the corruption that exists, and truly make those who are being oppressed free," the group claimed.
The Justice Department arrested five alleged leaders of Anonymous earlier this year after one of the hackers became a government informant in exchange for a lighter sentence.