Almost all writers of detective fiction write in the first person. I'm not sure why, but suspect it's to avoid the challenge of hammering out details regarding how significant case information is developed. Note, for instance, how most non-P.I. authors conveniently have a police source who comes across with background information for the hero, some old chum from yesteryear.
It isn't explained how the information was developed. Having police sources is rare. Cops don't particularly care for, or assist P.I.s and there are departmental rules governing queries into certain databases.
These days, cops can check few records without leaving their fingerprints all over the place which makes their superior inquire into their activities. They need a reason, i.e: a case file, to probe certain indices. In most police departments, cops can't even make long-distance telephone calls without first completing a call sheet explaining why and to whom the call is necessary, which becomes part of a permanent record.
I've never read a detective thriller that satisfied me once the writer lost credibility. If a writer hasn't been a P.I. or isn't consulting closely with one, it's easy to spot holes in a manuscript. I'm not a surgeon or a rocket scientist, so I certainly wouldn't attempt a detailed medical or scientific drama without a qualified co-author or consulting competent authorities. Screenwriters can evade scrutiny with vague explanations; novelists can't.