Nucor Corporation, the largest steel producer in the United States, has experienced a significant cybersecurity incident that forced the company to take portions of its network offline and implement emergency containment measures. The attack has temporarily halted production at multiple facilities across the company’s operations.

In an 8-K filing submitted to the U.S. Securities and Exchange Commission (SEC), Nucor disclosed that unauthorized third-party access was detected in certain information technology systems. The company immediately activated its incident response plan, proactively disconnected potentially affected systems, and implemented containment and remediation measures.

“Upon detecting the incident, the Company began promptly taking steps to contain and respond to the incident,” stated the SEC filing. Nucor has also notified law enforcement authorities and engaged external cybersecurity experts to assist with the investigation.

As a critical infrastructure supplier, Nucor employs over 32,000 people across numerous facilities in the U.S., Mexico, and Canada. The company is North America’s largest steel producer and scrap recycler, serving as a primary supplier of reinforcing bar used extensively in buildings, bridges, roads, and other infrastructure projects. In the first quarter of this year, Nucor reported revenue of $7.83 billion.

While the company has begun gradually restarting production operations, the full impact on Nucor’s business remains unclear. No details regarding the date or type of attack have been provided, leaving questions about whether the incident involved data theft or encryption. At this time, no ransomware groups have claimed responsibility for the attack.