Chicago police said a man was kneeling in the middle of the frozen lagoon, and an officer was trying to help him to safety about 5:30 p.m.
But, in the process, the ice cracked, and they both fell in.
Other responding officers were able to get the officer and victim to safety.
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"We're here to remind the public, as the weather begins to change, stay away from any large body of water," an officer said. "Do not venture off into the ice; it may look safe, but it's not."
Police recommend ice rinks as a safe way to enjoy the ice.