Cyber attacks on companies maintaining U.S. critical infrastructure have multiplied over the past three years by more than 20 times, CNN reports.
A recent Department of Homeland Security report says that 198 incidents were reported in 2011 compared to nine in 2009.
Response teams for cyber emergencies deployed to the location of the attacks 17 times to investigate the attacks, the report said.
Spear-phishing is the most common attack method, where attackers try gaining access to critical information by uploading malicious attachments into the system, according to CNN.
The Industrial Control Systems Cyber Emergency Response Team said companies involved with power grids, water filtration and nuclear facilities were among the main targets, according to the report.