The FBI must pay the legal fees of Muslim activist groups that sued the federal agency for access to its files, according to a U.S. District Court ruling filed Thursday.

Judge Cormac Carney made clear that the financial sanction was not based on the merits of the Islamic Shura Council of Southern California's Freedom of Information Act case, but it was to punish a government that chose to lie to its own judicial system.

"The Court must impose monetary sanctions to deter the Government from deceiving the Court again," Carney wrote. He gave the Islamic Shura Council 14 days to provide an affidavit detailing its costs.

After a nearly five-year court battle, Carney ruled in April that the council could not review additional records of FBI inquiries into its activities, but he berated the government for misleading the court about the existence of the files.

"Parties cannot choose when to tell the Court the truth. They must be truthful with the Court at all stages of the proceedings if judicial review is to have any real meaning," Carney wrote.

FBI spokeswoman Laura Eimiller declined to comment on the ruling, noting that the agency does not comment on litigation and has not commented on this case.

The ruling refutes the FBI's claim that admitting the files existed would have compromised national security, noting the privacy of sensitive filings can be protected by the courts.