Norfolk Southern believes a software defect — not a hacker — was the cause of the widespread computer outage that forced the railroad to park all of its trains for most of a day earlier this week.

The railroad said Friday that it traced Monday's problem to a defect in the software one of its vendors was using to perform maintenance on its data storage systems.

Both the railroad's primary and backup systems became unresponsive at the same time. The update was made to one system and then automatically copied to the other system allowing the defect to spread. Norfolk Southern didn't identify the vendor except to call it "a leading global technology provider."

The railroad, based in Atlanta, reiterated that it has found no evidence that the outage was caused by “an unauthorized cybersecurity incident.”

Norfolk Southern said it has been making progress in clearing up the backlog of trains that accumulated while its network of nearly 20,000 miles of track in the Eastern U.S. was shut down. The railroad has been working to keep its customers updated on their shipments, but it has said the effects of the outage could linger for a couple weeks.

Regulators have been scrutinizing Norfolk Southern's operations ever since a fiery February train derailment in Ohio forced thousands of people to evacuate their homes because of concerns about the toxic chemicals the train was carrying. The cleanup from that derailment is ongoing.

A package of railroad safety reforms members of Congress proposed after the Ohio derailment has stalled in the Senate and has yet to get a hearing in the Republican-controlled House.