Stephen Peterson's lawyer said Saturday afternoon that they were disappointed but not surprised by the decision.
The Oak Brook Board of Fire and Police Commissioners ruled that he failed to disclose information to law enforcement about the disappearance of his father's wife Stacey Peterson.
Peterson was cleared of other charges, including possession of an unlawful weapon and failing to keep an internal investigation confidential.
"Officer Peterson's continuing claim the weapons and money were irrelevant to the investigation by the Illinois State Police were self-serving, disingenuous and not credible," said board chairman Frederick Cappetta.
"He's been very tough. He's really lucky he's got most if not all of the police officers on the department supporting him. He's got friends to support him," said Peterson's attorney Tamara Cummings.
Stephen Peterson's lawyer says he will appeal the firing. She says the goal is to get him back on the job "where he belongs."
The Associated Press contributed to this report.