Hackers have tried to break into the email accounts of a select number of Washington Post journalists, according to an internal Washington Post memo obtained by CNN.

The Post discovered the “possible targeted” hack of its email system last Thursday, prompting the newspaper to reset login credentials for all its employees on Friday, Washington Post Executive Editor Matt Murray said in a memo Sunday to employees.

“Although our investigation is ongoing, we believe the incident affected a limited number of Post journalists accounts, and we have contacted those whose accounts have been impacted,” Murray said.

“We do not believe this unauthorized intrusion impacted any additional Post systems or has had any impact for our customers,” he added.

It was not immediately clear who was responsible for the hack. Journalists are regular targets for both state-backed spies, who are interested in tracking their reporting before it becomes public, and cybercriminals, who are interested in extorting news organizations.

A spokesperson for The Post declined to comment when asked who might be responsible for the hack.