The U.S. Department of Homeland Security says there was a huge rise in the number of public utilities and nuclear energy companies reporting cyber attacks last year.

A new department study says authorities received 198 reports of cyber security threats in 2011, compared to just nine in 2009.

Many of the threats were carried out against public water companies, nuclear energy providers and electric power grids.

Homeland Security says that among the reports was a virus attack against a nuclear energy facility. The virus entered the system when an employee inserted an infected portable computer drive into the network to download some documents.

Homeland Security officials say the jump in the number of reported attacks is partially because many firms are more willing to contact federal authorities than in the past.