Police said a hand grenade was found by a resident Tuesday, and then more "explosive material" was found Friday.
In both instances, the Highland Park Police Department and the Waukegan Bomb Squad said the explosives were removed in a safe and secure manner.
Police said Openlands is closing the entire area to the public until further notice out of "an abundance of caution."
In a Facebook post Openlands said it's "taking steps to ensure public safety by alerting the public and working with the military."
Fencing and signage has been placed around the affected area.
"Due to the ongoing investigation, we do not have further comment at this time, but hope you will help inform the public in the spirit of public safety and transparency," Openlands said.
It is unclear if the material is a replica, according to officials, but tests are pending.
People are being urged to keep off the bluff and lakefront until an investigation is complete.