Hackers working on behalf of the Chinese government broke into the computer networks of at least six U.S. state governments in the last year according to a report released Tuesday by private cybersecurity firm Mandiant.

In this case, the report says, the hackers exploited a previously unknown vulnerability in an off-the-shelf application used by 18 states for animal health management.

In addition, they exploited a software flaw known as "Log-4-j" that was discovered in December and possibly present in hundreds of millions of devices.

The report does not identify the compromised states or offer a motive for the intrusions, which began last May and continued through last month.

But the Chinese group believed responsible for the breaches, A-P-T-41, is known to launch hacking operations both for espionage and for financial gain.