The FBI and the Cybersecurity and Infrastructure Security Agency (CISA) disclosed Tuesday that multiple U.S. natural gas and oil pipeline companies were successfully breached by Chinese hackers for two years beginning in 2011.

The agencies outlined the campaign, which ended in 2013, in a joint cybersecurity advisory released Tuesday. The agencies noted that 13 companies were successfully breached, three were described as “near misses” and eight others were subject to an “unknown depth of intrusion.”

CISA and the FBI attributed the incidents to Chinese state-sponsored hackers and noted in the advisory that it was a targeted attack likely designed to further develop China’s cyber capabilities.