Hackers attempted a targeted cyberattack on the email accounts of several Washington Post journalists, according to an internal memo. The breach was discovered on Thursday, leading the paper to reset all employee login credentials by Friday.

While the investigation continues, the intrusion appears to have affected only a limited number of journalist accounts, with no impact on other systems or customers, said Executive Editor Matt Murray.

The source of the attack is unknown, but journalists are often targeted by state-backed hackers and cybercriminals seeking sensitive information or leverage. The Wall Street Journal, which reported on the incident, previously suffered a years-long hack by suspected Chinese operatives.