The intrusion into the federal judiciary’s case filing system was like “taking candy from a baby,” said one person with knowledge of the hack.

Investigators have uncovered evidence that Russia is at least in part responsible for a recent hack of the computer system that manages U.S. federal court documents. Some of the documents in the system are highly sensitive records that could reveal the sources behind evidence gathered in the prosecution of national security crimes.

Evidence suggests that Russia is partly responsible for a recent hack of the federal court records system, which may have exposed sensitive information about criminal cases and confidential informants, according to a report.