Norfolk Southern railroad believes a software defect — not a hacker — caused a widespread computer outage Monday that disrupted operations.

The Atlanta-based railroad announced Friday that after completing an analysis of the outage, it determined that it was caused by “a routine maintenance procedure” performed by a vendor. No evidence indicated hat it was related to a “cybersecurity incident," said the company. 

The outage led to several delays across the Midwest and East Coast, including for passenger trains operated by Amtrak.

The company’s data storage system is overseen by an outsourced technology provider. During maintenance, a defect in the vendor’s software spurred an error that rendered the railroad’s primary and recovery data storage systems unresponsive.

Norfolk Southern didn’t identify the vendor, but referred to the company as “a leading global technology provider.” 

“We are working closely with the vendor to understand the steps they are taking to strengthen their systems to prevent this incident from reoccurring and identify opportunities for Norfolk Southern to become more resilient,” Norfolk Southern said. 

The railroad’s tech, marketing and operations teams worked around the clock to restore service, the company said.

There remained on Saturday a temporary backlog of trains while its operations were shut down. But Norfolk Southern, which operates 19,300 route miles, expects a full recovery in the coming weeks. 

Norfolk Southern last made headlines in February after a fiery train derailment in East Palestine, Ohio.