I think you’re mistaking “investigation” with “prosecution” or something else. The cops can investigate literally anyone. You can hire a PI to investigate anyone.
There are limits however on what investigative actions can be legally taken by cops based on the evidence they have. Even with no evidence, they can still do things like interview people who know the POI, even follow them around in public. They can’t, for example, detain them and beat a confession out of them, or search their house willy nilly.
If we’re going to get technical, yes, there are extremely limited forms of investigative action that could be taken based on a broad suspicion. But without “reasonable grounds” the police are forbidden from interviewing suspects, detaining people, or performing any form of search or seizure. That’s not an investigation, that’s walking up to a guy in the street and going “Yo, did this guy do any crimes?” What on earth do you imagine would come out of that beyond wasting police time?
As for your comparison with private detectives, do I really have to explain that constraints on state power only apply to state actors? Private detectives are, by definition, private individuals. And they’re still basically constrained in all the same ways anyway, because you can’t just break into someone’s house or hack their computer. I know PIs in Canada. 100% of what they investigate falls into exactly two camps; infidelity, and insurance fraud. That’s it. They’re not Sherlock Holmes.
I think you’re mistaking “investigation” with “prosecution” or something else. The cops can investigate literally anyone. You can hire a PI to investigate anyone.
There are limits however on what investigative actions can be legally taken by cops based on the evidence they have. Even with no evidence, they can still do things like interview people who know the POI, even follow them around in public. They can’t, for example, detain them and beat a confession out of them, or search their house willy nilly.
If we’re going to get technical, yes, there are extremely limited forms of investigative action that could be taken based on a broad suspicion. But without “reasonable grounds” the police are forbidden from interviewing suspects, detaining people, or performing any form of search or seizure. That’s not an investigation, that’s walking up to a guy in the street and going “Yo, did this guy do any crimes?” What on earth do you imagine would come out of that beyond wasting police time?
As for your comparison with private detectives, do I really have to explain that constraints on state power only apply to state actors? Private detectives are, by definition, private individuals. And they’re still basically constrained in all the same ways anyway, because you can’t just break into someone’s house or hack their computer. I know PIs in Canada. 100% of what they investigate falls into exactly two camps; infidelity, and insurance fraud. That’s it. They’re not Sherlock Holmes.