ThyssenKrupp, one of the world’s largest producers of steel, revealed Thursday that it had been targeted by attackers located in Southeast Asia engaged in what it referred to as “organized, highly professional hacker activities.”

According to the company, technical trade secrets were stolen from the steel production and manufacturing plant design divisions of the company. The breaches were discovered by the company’s internal security team in April and traced back to February. Hackers stole project data from ThyssenKrupp’s plant engineering division and from other areas yet to be determined, the company said.

The attacks hit sites in Europe, India, Argentina and the United States run by the Industrial Solutions division, which builds large production plants, reported German business magazine Wirtschafts Woche. The Hagen Hohenlimburg specialty steel mill in western Germany was also targeted, the report added.

No breaches were found at its marine systems unit, which produces military submarines and warships. The infected computer systems have been fixed and are now protected from further cyberattacks by continuous monitoring.

The company did not identify specific locations which were infected or why it had not previously disclosed the attack.