In Justine Damond shooting, took them many months to even charge him. Here they are already talking about manslaughter charges. And in some cases where relative kills another relative due to thinking a stranger broke in, there are no charges at all. By the way they can't interview her unless she agrees to the interview, just as regular citizen.
If they treated this as an officer involved shooting, there are department protocols that are followed. Usually departments use the International Association of Chiefs of Police (IACP) as a template. Bear in mind that the officer involved in a shooting should be relieved from any role at the shooting site as soon as other officers arrive. Here is what the the IACP and DOJ put out on officer involved shootings after supervisors and peers arrive:
"Even if the involved officer does not have obvious injuries, it is good practice to transport any involved officer to an emergency medical facility. Medical personnel can then evaluate and monitor the offi- cer(s) for delayed physical or emotional reactions that may require medical attention. At the same time, investigators should ensure that testing is performed to document whether alcohol or drugs are in
the officer’s system, if such testing is not prohibited by law or union contract. Some agencies do not require testing for drugs or alcohol without reasonable suspicion; some contend this is an overt or subjectively accusatory action, which conveys the agency’s lack of trust in in the officer or suspicion of wrongdoing. These are legitimate concerns. However, medical personnel can perform tests for alcohol and drugs easily, and these tests are a part of any thorough investigation of an officer-involved shoot- ing. Additionally, the results can serve to counter any contention that such substances impaired the officer’s judgment. "
http://www.theiacp.org/portals/0/documents/pdfs/e051602754_Officer_Involved_v8.pdf
If not treated as an officer involved shooting, most people who shot someone are taken to the station and then can determine that they will not speak to the police or ask for a lawyer. If a person who shoots someone is suspected of being altered, the police usually move to immediately get a warrant for testing for substances.
Either way, it seems that this officer was indeed treated outside of usual protocol. The question remains why.
I am glad that the Texas Rangers are in charge of this investigation because the department has already made some large mistakes here in investigating their own.