In a private notification sent Friday to some congressional offices, obtained by NBC News, the Library of Congress said that hackers “accessed email communications between congressional offices and some Library staff, including the Congressional Research Service.” The hackers had access from January until September, the notification said.

An investigation is underway after a "foreign adversary" hacked the Library of Congress email systems earlier this year.

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,

Affected staff were notified Friday afternoon, according to an internal email. Capitol Hill communications with the Congressional Research Service frequently involve confidential legislative drafts or policies still in the brainstorming stage.

The hacker is described as an "adversary," and the emails in question were between library staff and staff for members of Congress.