Nuclear power facilities have been among the targets of multiple hacking attacks in the United States since May, disclosed in a joint report issued by the Department of Homeland Security and the FBI, the New York Times reports.

Other energy and manufacturing companies have been victim of the malware attacks, which have reportedly been in the form of spear phishing attempts on the personal computers of employees working for the plants. There is no evidence that the hacking attempts have penetrated control systems of the facilities themselves.

One such facility as identified in the report is a nuclear power plant in Burlington, Kansas, owned by the Wolf Creek Nuclear Operating Corporation. The company has said that the internal networks which actually keep the plant functioning are separate from the corporate network over which it conducts business.

The DHS-FBI report concludes that these hacks have been more like along the lines of a reconnaissance attempt to map out networks in advance of a possible future malicious attack.