The bulletin, obtained by CNN, says disgruntled current and former employees have in the recent past been able to use their insider knowledge to disrupt operations at water, gas and waste facilities.

It also notes that if "violent extremists" were able to "gain access to an insider or acquire an insider position, this increases the likelihood of success and impact of an attack."

The bulletin did not talk about a current threat to these sensitive facilities, CNN said. But the document did detail a case in 2010 when a U.S. citizen was arrested in Yemen during a round-up of suspected al Qaeda members. The person had worked "for several contractors performing non-sensitive maintenance at five different U.S. nuclear power plants from 2002 to 2008," the bulletin said.

"While DHS has no specific, credible intelligence of an imminent threat posed to the private-sector utilities, several recent incidents highlight the ongoing threat to infrastructure in the utility sectors from insiders and outsiders seeking facility-specific information that might be exploited in an attack," DHS spokesman Matthew Chandler said.

The department will work closely with state and local officials including utility companies "to take steps to best protect from potential threats, including protecting our nation's infrastructure," he said.