Several U.S. newspapers came under attack from apparent hackers on Saturday, preventing some from printing and distributing their daily editions.
The Los Angeles Times, San Diego Union-Tribune, Chicago Tribune and Baltimore Sun were among the major newspapers that had printing issues on Saturday. The cyberattack appeared to target newspapers owned by the Tribune Publishing Company.
"This issue has affected the timeliness and in some cases the completeness of our printed newspapers," Tribune Publishing spokeswoman Marisa Kollias said in a statement. "Our websites and mobile applications however, have not been impacted."
Most tweeted online apologies to readers -- as well as links to stories on their sites explaining the problem.
The Department of Homeland Security told ABC News in a statement that it is investigating.
"We are aware of reports of a potential cyber incident effecting several news outlets, and are working with our government and industry partners to better understand the situation," Katie Waldman, DHS spokeswoman, said.
The source of the attacks was a "foreign entity," according to a report in The Los Angeles Times.
It is not clear what the motive of the attacks was, but the Tribune Publishing Company said in a statement "the personal data of our subscribers, online users, and advertising clients has not been compromised."
The Trump administration released a new National Cyber Strategy in September aimed at deterring malicious online behavior, specifically from Russia and China.
The paper accuses Russia, Iran and North Korea of conducting “reckless cyber attacks that harmed American and international businesses and our allies and partners without paying costs likely to deter future cyber aggression.”
ABC News' Lee Ferran and Amanda Maile contributed to this report.