Independence Grove Forest Preserve near Libertyville is expected to remain closed Friday as the investigation continues into a bomb threat that has shut down activities this week, authorities said.
Julie Kenney, a public information representative for the Chicago division of the U.S. Postal Investigation Service, said no explosive device had been found as of Thursday afternoon.
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"The investigation is not over. They're still going to be searching," Kenney said. She declined to say what that entails.
Authorities have been questioning people regarding the situation, which involved "multiple" letters sent to the Lake County Forest Preserve District and undisclosed recipients, she said.
The letter received by the forest preserve district specified the target of the threat as a privately-hosted event scheduled at the preserve, not a district function, according to an update issued Thursday evening by the district.
"No arrests have been made, but there are people of interest. They have been interviewing three that I know of. There's nobody in custody," according to Kenney.
She said a determination will be made Friday whether Independence Grove on Route 137 will remain closed.
The U.S. Postal Investigation Service is the lead agency, with assistance from the Lake County Forest Preserve Ranger Police and the Cook County Sheriff's Bomb Unit.
Ranger Police Chief John Tannahill again stressed early Thursday the preserve will remain closed indefinitely until the investigation is completed. He has said Independence Grove will reopen when authorities deem it safe.
The ranger police are taking "every necessary precaution to secure the site until the threat can be nullified," according to the district release.
Ranger police closed the district's fourth largest and most popular preserve Tuesday evening before a scheduled 7:30 p.m. concert at the Millennia Plaza amphitheater. The concert was canceled and anyone in the preserve at the time was told to leave due to a matter of public safety.
Activities such as camps that had been planned at Independence Grove have been canceled, diverted or rescheduled, officials have said.
The Independence Grove Dog Exercise Area and Des Plaines River Trail are open and unaffected by the closure.
• Daily Herald staff writer Lee Filas contributed to this report