Did you read the article? Ferguson PD are required to file a Use of Force Report. This is not related to who does any investigation. Their own policy says that the sergeant on duty that day should have filed one, but he did not. In other words, prior to the investigation, there are many pieces of paperwork, all subject to FOIA, that the FPD should have produced but didn't.
That's not the correct interpretation.
Missouri Revised Statutes 610.100.1(5) defines the term investigative report as:
a record, other than an arrest or incident report, prepared by personnel of a law enforcement agency, inquiring into a crime or suspected crime, either in response to an incident report or in response to evidence developed by law enforcement officers in the course of their duties.
610.100.2 controls when these records become public:
Each law enforcement agency of this state, of any county, and of any municipality shall maintain records of all incidents reported to the agency, investigations and arrests made by such law enforcement agency. All incident reports and arrest reports shall be open records. Notwithstanding any other provision of law other than the provisions of subsections 4, 5 and 6 of this section or section 320.083, investigative reports of all law enforcement agencies are closed records until the investigation becomes inactive...
Regardless of any other provision of law, all investigative reports into this use of force are closed until:
(a) A decision by the law enforcement agency not to pursue the case;
(b) Expiration of the time to file criminal charges pursuant to the applicable statute of limitations, or ten years after the commission of the offense; whichever date earliest occurs;
(c) Finality of the convictions of all persons convicted on the basis of the information contained in the investigative report, by exhaustion of or expiration of all rights of appeal of such persons;
(b) Expiration of the time to file criminal charges pursuant to the applicable statute of limitations, or ten years after the commission of the offense; whichever date earliest occurs;
(c) Finality of the convictions of all persons convicted on the basis of the information contained in the investigative report, by exhaustion of or expiration of all rights of appeal of such persons;