Criminal investigations of police officers have delays that are formally built in through union contracts and police protection laws

Minneapolis police officers involved in “critical incidents” — which include situations where a person dies in police custody — must be given “a reasonable opportunity” to consult with a lawyer before they’re asked to give a voluntary statement to an investigator, according to the labor contract between the city and the police union that expired at the end of 2019.

Some more info on this from checkthepolice.org:

50 cities and 15 states restrict interrogations by limiting how long an officer can be interrogated, who can interrogate them, the types of questions that can be asked, and when an interrogation can take place.

41 cities and 11 states give officers under investigation access to information that civilians suspects don't get, including 16 cities that allow officers to review all evidence against them prior to being interrogated.

64 cities and 10 states limit disciplinary consequences for officers, for example by preventing an officer's history of past misconduct from being considered in future cases, and/or limit the capacity of civilian oversight structures or the media to hold police accountable. This includes 48 cities that let officers appeal disciplinary decisions to an arbitrator who has the power to reinstate that officer and who in many cases is selected, in part, by the accused officer or the police union.