"Understanding the amount that the FBI deemed appropriate to spend on the tool, as well as the identity and reputation of the vendor it did business with, is essential for the public to provide effective oversight of government functions and help guard against potential improprieties," said the suit, filed in U.S. District Court for the District of Columbia under the U.S. Freedom of Information Act.

In rejecting earlier requests to divulge the information, the government said revealing the records could affect "enforcement proceedings," but did not elaborate.

The FBI's announced in March that it had purchased a tool to unlock the iPhone, aborting the court fight with Apple that had in turn triggered a debate about the proper balance between electronic privacy and national security.

At the Justice Department's request, a magistrate judge in February directed Apple to create software that would bypass security features on the phone so that the FBI could get into the device and scour it for potential evidence. Apple contested the order, saying the FBI's demand set a dangerous precedent and could undercut security protections for its customers.

The two sides were headed for a court showdown when Justice Department officials revealed that a party outside the U.S. government brought it a potential solution to unlock the phone. The FBI said a week later it successfully unlocked the phone using the tool. The suit cites media reports as saying investigators did not find any links to foreign extremist groups.

The FBI has not disclosed how much the tool cost or revealed how it worked. It also refused to share the information with Apple, which had expressed concern that circumventing its security protections could compromise its products.