Say someone tips off the feds that John Doe committed a murder “somewhere, someplace, sometime.” The feds have nothing to go on, but must do something, so they start querying everyone on John’s Facebook friend list, every colleague at work, every family relative, etc. “do you know anything about this?”
Do they need a warrant to go prying about like this, or is this totally open-source info for them as long as it’s not “formal” interrogation?
Surely, you or I could go around asking those questions, too? Why would one need any particular authorization to ask questions?
Asking people questions doesn’t require any legal authority. A formal interrogation doesn’t require a warrant either. Warrants are for searching people’s stuff. Asking them things doesn’t require anything in particular. If they want to be able to use what you say as evidence in a legal proceeding, they need to mirandize you first.
Law enforcement gets all kinds of “tips” all the time, much of it totally bogus, and I assure you that if you report that someone killed someone, somewhere, they would very likely totally ignore it as non-credible.
Procedurally, a police officer or detective generally has to have an opened investigation to go around investigating or bringing people in for questioning. However, this is a policy standard of their department, not a legal requirement, and while going around asking misleading or prying questions without a reasonable basis for sufficient could be taken as harassment or potentially slander, I think that would be very difficult to make that stand up in court.
Not necessarily. If you are placed under arrest, then police have to read a Miranda warning at peril of having all disclosures be inadmissible. However, if you are not under arrest and voluntarily let slip some detail of culpable information, it is potentially admissible (depending upon circumstances including whether the person being questioned felt threatened or was under the impression that you had been arrested).
There are warrants for different purposes but per you are correct that there is no “warrant for questioning”. The police do have the authority to detain you if they have reasonable suspicion that you are a witness or participant to criminal activity but only for “brief and cursory” questioning or if they believe you pose a risk of flight or harm (although in some jurisdictions this can amount to a detainment of many hours). If there is any question you should clarify your status by asking “Am I being detained?”, “Am I being arrested?”, and if so “For what cause?”
You are, of course, not obligated to talk with or respond to questions to the police or other interrogators per your right against self-incrimination (i.e. “taking the Fifth”), although if this impedes an investigation it could potentially be considered to be deliberate interference or even accessory after the fact (which is unlikely to stand in a court but could be threatened by a detective or prosecutor to get you to talk). In any non-cursory interaction with police, and especially if you are detained or arrested, you should give serious consideration to asking for legal representation before answering any questions because even something that totally seems like an innocuous comment to you could be taken as some kind of evidence of crime or conspiracy. My cite is the American Bar Association’s #3 legal film of all time.
Stranger
Huh? AFAIK (IANAL) there is not compulsion for anyone any time to answer any question, and that is not illegal. Police can stop you to ask questions, but you can politely decline and go about your business. If the police have “probable cause” (any lawyers to chime in about the exactitude here?) they can detain you for questioning. I think to take you to the station, as opposed to a brief detention, they need to arrest you. To arrest you they need more definite reasons. In all cases, you always have the right to remain silent (and if arrested, ask for a lawyer).
that’s why on the cop shows, they always say “if you ask for a lawyer, we (the police) can’t help you any more.” the police never “help” you anyway. they cannot even promise any deals.
It can be illegal to make false or misleading statements that attempt to derail an investigation. If put on the stand during a trial (or during a Grand Jury fishing expedition in the USA) you may invoke the fifth to avoid revealing personally incriminating evidence, but otherwise must answer any questions.
It’s “reasonable suspicion” for an investigatory detention. Probable cause for an arrest.
But investigatory detention, as I understand, is basically stopping you on the street for a few minutes to ask questions? And taking you to the station against your will is arrest?
And to get to the OP’s question - the police must have a “reasonable suspicion” to investigate a crime. As i understand, this would be a complaint, but there must be some objective reason (facts) to continue. If someone were to call 911 and say “this person is threatening me and my dog” and the police, on attending and asking questions, do not see anything that looks like threats or a threatening attitude, no facts to back up the complaint, I presume they have no grounds to continue investigating anything except maybe a false complaint. (Which would be borne out if the facts they did find appear to do so). They are not allowed to simply target someone, and then proceed to investigate all their (legal) activities and try to find anything they can that rises to the level of chargeable offense.
I’m pretty sure they are allowed to, by law. All the investigatory standards in law are about interfering with a suspect. You can’t go through their stuff without a warrant. You can’t interfere with their movement (arrest) without a certain standard of evidence to do so. You can’t go on their private property, etc.
But you can follow their car around on public streets and wait for them to fail to signal a turn. You can park a cruiser in front of their house and take photos of everyone who comes and goes, or in through their open windows. You can do all sorts of things to try to find evidence of a crime.
At some point those actions may rise to the level of harassment, but there’s generally pretty wide latitude given to law enforcement officers for this kind of thing.
Doing so without reason is harassment. The problem isn’t whether or not it is, the problem is getting the rest of the authority structure to recognize it and do something about it.
Without reason, or with a corrupt reason, sure. Without reason that is legally articulable into a given standard of proof, maybe not.
Police or other investigators cannot legally compel you to respond to questions or penalize you just for refusing to respond–that authority rests with the court and its power of witness summons (subpoena ad testificandum, although even that cannot generally be used to compel self-incriminating testimony unless immunity has previously been granted)–but they can detain you for “brief and cursory” questioning, or if they have reasonable suspicion that you may have participated in the crime or are a material witness to a crime. What police cannot do is stop some random person with no cause or reasonable suspicion, or demand anything from you beyond a verbal identification without cause relating to a crime.
The police can detain you for a period sufficient to ask questions and determine whether probably cause exists to affect an arrest, and can invite you voluntarily to “come down to the precinct” (or wherever) to answer questions. Involuntarily taking you into custody without probable cause to for arrest is involuntary detention and is a clear violation of your civil rights, although police will sometimes imply that you are required to participate or will suffer penalty for not answering questions. Again, if you are material witness to a crime and refuse to provide pertinent information in a timely fashion that interferes with an investigation or permits the perpetrator(s) to evade custody, you could theoretically be charged with aiding and abetting after the fact but that’s more of a hollow threat to encourage you to comply rather than a charge that a prosecutor is likely to pursue unless it facilitates their primary case.
Actually, they are, and frequently do, particularly if they are directed to do so by a prosecutor. A police officer or investigator can follow you around in public areas and maintain visual and auditory surveillance as long as they are doing so in a way that is not a violation of your privacy (e.g. peaking through your windows or over a fence, bugging your house or using other means to gain access to private spaces, et cetera). They can search your trash after you’ve rolled it to the curb, photograph you in public, question associates and coworkers, et cetera. If they are doing so with a clear agenda of harassment or interference (e.g. trying to make the suspect appear guilty or cause them hardship through unemployment, et cetera) and without any reasonable suspicion then the suspect can issue a cease and desist letter with a threat to file a complaint for harassment, but that is also more of a legal jousting than an effective threat of action unless the police have actually caused some financial injury.
Stranger
That last part is a gray area. They can surveil activities that are “in plain view” from someone on the street, but if they were using some kind of enhancement like a telephoto lens to view inside of private property that would likely not be allowed without a warrant authorizing photographic or electronic surveillance. Taking pictures of people coming and going on a public street, on the other hand, is completely legal as you have no expectation of privacy in public spaces, and in fact cars are deliberately identified by alphanumeric tags specifically to facilitate identification.
Stranger
Police can take aerial photos without a warrant, so I doubt that peeking over your fence (if they could do it while standing on public property) would require one.
That’s another kind of gray area; technically without a warrant helicopters and drones are only supposed to be surveilling public areas like streets, but if they “happen” to see criminal activity on private property “in plain view” then it is actionable and admissible. What they can’t do is deliberately hover above your property waiting for something to happen absent any cause to believe a crime is in commission. The reality, on the other hand, is that it is pretty easy to skirt that distinction, especially if you are a burnout cop with access to a top secret helicopter program build for stealth surveillance.
Stranger
That’s not what the Supreme Court said in California v. Ciraolo
Nothing about only observing public areas. It’s about observing from public areas. In which apparently “the sky” is included.
Fair enough. I think you’d still get some complaints if you hovered your helicopter over someone’s house for hours on end. 
Stranger
Just think how many Columbo episodes would have ended without a conviction if the perps had just SHUT UP.
Same for The Closer.
As always, it’s worth linking the entertaining and edifying Don’t Talk to the Police video.
The police officer mentions tactics the police use to get you to talk without having to Mirandize you. Specifically, they only have to mirandize you when they ask you questions. If they sit in a room with you and let you start talking, that’s not an interrogation.
Definitely 
For clarification, I don’t mean the police asking questions of the perp, but rather, his friends or acquaintances. Because even asking questions can harm reputation. Say the cops ask all of John Doe’s friends if he is known to be an arsonist. Even if John is innocent, the very question asking would plant the seed in people’s heads.
I think it’s a little hard to answer, because asking for no reason at all would tend to indicate some kind of intent to harass, and it’s hard to imagine a scenario where that isn’t the case.
But, basically, police, just like anyone else, can engage in conversations. You or I could ask someone’s friends and neighbors about the person’s whereabouts and habits, etc. The questions might be considered odd, but as long as you aren’t telling the friends and neighbors that the person did anything, or implying that you know they did, I don’t think it would cause legal trouble.
While it often is not focused on a specific person, an example is that police do this when they canvas an area after a crime. They may start by just knocking on doors and asking if anyone observed anything. At some point, if a person or vehicle of interest emerges, then they might be asking about whether anyone knows anything about that person or vehicle. The information they want includes exculpatory information to rule out involvement. (“Oh, yeah, my neighbor had rented that white van and parked it across the street. We helped him move his new fridge in and the old one out.” Or, “Yeah, that neighbor has been acting strange since the day of the crime…”)