An unidentified “adversary” broke into the email system for Library of Congress, accessing emailed information between January and September 2024, the Library said.

The library shared news of the “cyber breach” with some congressional offices, and NBC News obtained a copy of the message Saturday morning.

Law enforcement and the Capitol are investigating the hack. It is not yet clear who the adversary is or when exactly the hack took place.

The Library of Congress, which manages the world’s largest collection of books, photographs, records and other media, is located on the grounds of the US Capitol complex. US lawmakers and their staff use the library to research laws and the legislative process.

However, the House and Senate use different email networks than the Library of Congress, and the library says those networks “were not compromised in any way.”

The US Copyright Office system was not compromised either, the library said in its message to Congress.

The only things affected were “e-mail communications between Congressional offices and some Library staff, including the Congressional Research Service,” sent between January and September of this year.

“The library has mitigated the vulnerability that the adversary used to access the environment and has taken steps to prevent such incidents in the future,” the memo sent to congressional offices said.