The weather was clear about 9:15 p.m. Sunday when one of the village's seven sirens went off for about three minutes, Shaughnessy said. Only a few people in the village have the ability to activate the sirens, but none of them had done so, the chief said.



About an hour later, all seven sirens began sounding, even though no village officials had activated them, Shaughnessy said.



The sirens kept sounding even after officials tried to shut them off, prompting officials to cut their power supply after about 30 minutes, Shaughnessy said. The sirens alternated between a sound warning of dangerous weather and a sound warning of a military attack, which Shaughnessy said he had never heard before.



The southwest suburb's police department was flooded with calls from concerned residents, the chief said. Although the weather was calm Sunday night, officials had activated the sirens two nights earlier when a storm packing winds of more than 60 mph moved through the area, he said.



Police and officials from the company that maintains the sirens have determined that no one authorized to activate the sirens did so on Sunday night, and that a mechanical malfunction was not to blame, Shaughnessy said.



That means the system was likely hacked, he said.



"We haven't eliminated anything, but we feel the system was compromised by an unauthorized person," Shaughnessy said.



Lemont's sirens are activated by a radio signal containing a unique code, Shaughnessy said. Police suspect someone made a copy of the signal and broadcast it, activating the sirens, he said.



Lemont police have notified the FBI and the Federal Communications Commission, Shaughnessy said.



Village officials also learned that Evanston's sirens sounded without being activated by the city on Saturday night, he said.



Shaughnessy said Lemont has taken steps to prevent the system from being hacked again, but he declined to provide details about the precautions. He said power was restored to the system Monday morning and that the sirens worked properly during a test today.



"They're ready to go," Shaughnessy said. "If there's bad weather tonight or anything like that, they're ready to go."