NAGOYA, Japan—Toyota Motor Corp. has resumed operations at its 14 Japanese assembly plants after they were forced to shut down due to a computer glitch. The facilities account for one-third of the automaker’s global output.

More than 25 assembly lines that produce some of Toyota’s most popular models were affected by the issue. According to Reuters, a computer system that processes just-in-time orders for vehicle parts broke down.

“It is our understanding that the malfunction of the system was not caused by a cyberattack,” claims a statement released by Toyota. “However, we will continue to investigate the cause. We would like to apologize once again to our customers, suppliers and related parties for any inconvenience caused by the suspension of operations.”