(CNN) - The Department of Homeland Security (DHS) is telling the country's law enforcement to keep an eye on utility companies, warning in a bulletin that disgruntled employees may pose a problem.
The bulletin said the employees have used their insider information to disrupt operations.
Trespassers planning attacks on gas, water and other power plants are one thing, but in a post 9/11 world, danger can also come from within.
This spring, a disgruntled employee at a Mesa, AZ wastewater plant allegedly managed to shut down a system that could have caused a massive sewage backup.
Authorities said the suspect made a bizarre 911 call.
"Um, I have basically taken the plant hostage," he said.
Last January, gas service was cut off to almost 3,000 customers in Illinois when a recently fired employee broke into a monitoring station and cut off a valve.
DHS has not linked either case of sabotage to al-Qaeda, but the bulletin refers to a New Jersey man suspected of ties to terrorism.
Sharif Mobley worked at five nuclear plants doing maintenance work, then moved to Yemen to follow al-Qaeda leader Anwar Al-Awlaki.
Mobley was named in a nuclear regulatory report after he allegedly expressed militant Islamic views. He admitted he admires Al-Awlaki, but denied any ties to terror, and has not been charged with any crimes.
Yet authorities singled out the case to warn utilities that al-Qaeda uses publications to encourage attacks on sensitive locations.
A U.S. official said this week's warning is not based on new intelligence information, and DHS said there is no credible or imminent threat.
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