"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 published by the Chicago Tribune.
The source gave no other details about the origin of the attack or the motive, describing the attacker only as a "foreign entity", The Times reported.
A computer virus disrupted production of The Baltimore Sun, Capital Gazette, Carroll County Times and other Tribune Publishing newspapers across the country, the company said Saturday. "We apologize to all of our readers for the inconvenience".
Tribune Publishing said in a statement Saturday that "the personal data of our subscribers, online users, and advertising clients has not been compromised". Technology teams made significant progress in fixing the problem, but were unable to clear all systems before press time.
A computer virus prevented most of the San Diego Union-Tribune's readership from waking up with a paper Saturday morning.
"This is bad. Tribune Publishing got slammed with a virus (the rumor is ransomware) that has hobbled production, including at the L.A. Times and San Diego Union-Tribune, which still use TPub services".
A cyber attack caused major printing and delivery disruptions on Saturday at the Los Angeles Times and other major US newspapers, including ones owned by Tribune Publishing Co TPCO.O such as the Chicago Tribune and Baltimore Sun.
On its Twitter account, Tribune Publishing Company acknowledged the issues, empathizing with customers affected.
The company said it first detected the malware on Friday, which hit papers sharing the same printing plant.
Malware has, over time, become more sophisticated and coordinated, involving more planning by networks of hackers who infiltrate a system over time, she said.