WASHINGTON -- The Library of Congress has notified lawmakers of a “cyber breach” of its IT system by an adversary, a hack of emails between some congressional offices and library staff, according to an email obtained by The Associated Press.

The library said that an adversary accessed email communications during the period from January to September.

The matter has been referred to law enforcement, the library said. Authorities gave no immediate information on the attacker, including whether their identity was known.

“The Library has mitigated the vulnerability that the adversary used to access the environment and has taken measures to prevent such incidents in the future,” the library said in the notification.

NBC News first reported the breach.

The Library of Congress operates across from the U.S. Capitol within the complex grounds. It is the largest library in the world, according to its website, and it holds “millions of books, films and video, audio recordings, photographs, newspapers, maps and manuscripts in its collections.”

The Library is also the main research arm of Congress, and the home of the U.S. Copyright Office.

In the notification about the breach, the library noted that the House and Senate's information technology networks, “including individual House and Senate email accounts, were not compromised in any way."

It said “an adversary accessed email communications between congressional offices and some Library staff, including the Congressional Research Service, compromising the information obtained in those emails.”

The notifications said the U.S. Copyright Office systems were not impacted by the breach.

The library said it is in the process of analyzing which email communications were accessed and would contact specific congressional offices and staff members with more information.