How would the FBI apprehend this suspect?

Generally it would be local law enforcement that would be responsible for handling an issue like this. The FBI would be called in if the suspect had ties to foreign terrorism, domestic terrorism, organized crime, etc.

It depends on the severity of the tip. If they have indicated to someone that they have a plan, and the means to carry it out, the police/FBI would boot down the door and apprehend the person. That's actually the safest resolution for the officers, the suspect, and the public. When you say the person is 'planning' ie. scouting out locations, stocking up on ammo, law enforcement would act immediately.

Here's a recent case of someone who was charged for threatening a mass shooting.
https://time.com/5655250/florida-man-threatens-mass-shooting/

There was another article with a similar case in which the FBI was involved, but the site wants me to pay a subscription.

If someone has made generalized threats, for example "I have gun, and some days IDGAF." The police would promptly respond to this. They may send several units to the house, or wherever the person is, but they would be there to talk to the person and asses their mental stability, and not there to solely arrest the person. Laws change from place to place, but the police might be able to confiscate all firearms, and ammunition, as well as have the individual undergo psychiatric evaluation.

The police probably wouldn't plan an elaborate take down. Public safety comes before officer safety, and the potential for a shootout on a public street seems like a place where things can go wrong.

It just isn't illegal for a person to be going through rocky times, and own firearms. Or if I know my neighbor has a gun, and I start to hear constant arguing from his house, not me, not the police can determine that this person is a threat to commit a mass shooting. It makes these cases difficult to prevent because a lot of the time the police, and medical professionals have their hands tied by an individuals legal rights.

In any case there would not be any 'wait and see', they would act immediately.

I don't know jack, but dawn raid with a SWAT team?

There are lots of people with large arsenal of weapons. As long as the individual weapons are not illegal and/or the person is not precluded from owning firearms, nothing illegal in that. Nothing illegal in posting pictures of your arsenal. Nor is it illegal to be fired. Lots of people are suicidal. It doesn't mean that the person meets the requirements for an involuntary commitment.

This would be local, not FBI. You might get the police doing a conversation with the guy, but you need a lot more than this for the police to go any further.

Now understand when the police show up at the door to discuss this with the guy, it can blow sky high.

Jim Clark-Dawe

Possible that your character's race, religion and ethnicity might all play a part in reactions as well.

Different police departments have different policies in place. As someone previously mentioned this is, in most cases, a local police matter. So how would the FBI handle receiving this tip? They would gather all the information from the tipster and call local law enforcement to handle it.

Local law enforcement has multiple options. If they believe that someone is suicidal and know where they are they will do a well-being check on this person. They may utilize Critical Incident Trained officers (CIT). This is a fairly well known program in the US. If the threat is specific they may send someone to the target location to interview people and make sure the suspect isn't already there. Local law enforcement may also try to meet the tipster in person to gain more information. At the very least there's a couple threat reports to be made.

As the previous commenter said, the nature and severity of the tip is important. Kicking down the door has a lot of other issues tied to it like forceable search and seizure. The tip itself would have to meet a lot of criteria for this to be a legal option for the police (I'm not saying the police may not bend the rule, but for it to be upheld...).

On a side note, stopping him in his car would be the least desirable. Traffic stops or "Terry Stops" are one of the most dangerous law enforcement activities and can put the general public in more danger.

Auteur said:

In my WIP, the FBI got a tip about someone who was planning a mass shooting. That person has posted pics on social media of his large arsenal of weapons and has recently lost his job in the tech industry, so he may be suicidal. He owns a house in a quiet neighborhood.

So, how would the FBI approach this situation? Would they knock on the door and try to ask him questions first, or would that be too dangerous? Would they surround his house and then knock on the door with their weapons drawn? Or would they wait for him to leave the house and then try to apprehend him in his car so he wouldn't have access to weapons other than what he had on his person? Or is there some other option?

Thanks in advance.

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What kind of tip? Someone owns guns, got fired, might be suicidal, doesn't = mass shooting or the FBI.

There are circumstances that'd bring the FBI in; there are circumstances that'd bring the ATF in; there are circumstances in which the 'tip' would lead to nothing, or to local law enforcement. It depends on a lot of factors not in your post. Where, who, when, why, etc., etc.

Auteur said:

Thanks everyone for your input!

I watched a video a few weeks ago about how the FBI apprehended a suspect who planted a bomb near a parade route. It didn't detonate, but it was still treated as an act of terrorism. After they found out who did it, they suckered him into leaving his house and then used construction equipment to stop him on a rural road in eastern Oregon (or it might have been Washington) because there would be less likelihood of a gun battle if the perp was away from his home arsenal. The action occurred back in the '80s, I believe.

So that was a response to an act of terrorism, and the situation in my novel is to prevent an act of violence. So, in this case, the police and FBI would work together on stopping it. But I guess they would work together in the first case, also. The FBI primarily does the investigative work and then they work together with the police to make the arrest, or engage in a massive firefight with bullets pinging off aluminum light poles and steel railings, and of course, somebody gets shot in the shoulder or maybe just a flesh-wound to the leg. And then the suspect escapes through the underground sewer system and comes up through a manhole under a van where he accomplices are waiting for him. I believe that's the typical scenario.

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Just addressing the beginning of the latter graph there, this is a very tricky thing.

Law enforcement doesn't, in general, prevent violence. That'd be nice, but it'd require reading minds and changing many, many laws and the basic building blocks of the current judicial system.

There's a huge difference between planting an actual device and, in your OP, doing.... nothing. There are situations in which le will act to prevent something, but the something (mostly) has to be really pretty clearly going to happen, serious, and some other things, and the acting can take many forms. This is all really kind of more specific and regulated than you'd think, I'd wager (just presuming you're not in law enforcement or mental health).

You threaten to harm the president, the Secret Service can show up and question you. You threaten someone random over a parking space, the cops will roll their eyes if someone calls them. You tell your psychologist you killed someone, they're not gonna rat you out. You tell them you fantasize about killing someone, they're probably not going to do anything involving law enforcement. You cross the Tarasoff line, they're calling a cop who will take that very seriously.

Same for all kinds of things -- there are levels and specifics and etc.

As to the apprehension question, that completely depends on, again, the specifics of the person, situation, etc. Trying to pull a traffic stop is a very bad idea, except in some very specific circumstances, but mostly no one is going to do that on purpose. It's hella dangerous.

Auteur said:

Thanks everyone for your input!

I watched a video a few weeks ago about how the FBI apprehended a suspect who planted a bomb near a parade route. It didn't detonate, but it was still treated as an act of terrorism. After they found out who did it, they suckered him into leaving his house and then used construction equipment to stop him on a rural road in eastern Oregon (or it might have been Washington) because there would be less likelihood of a gun battle if the perp was away from his home arsenal. The action occurred back in the '80s, I believe.

So that was a response to an act of terrorism, and the situation in my novel is to prevent an act of violence. So, in this case, the police and FBI would work together on stopping it. But I guess they would work together in the first case, also. The FBI primarily does the investigative work and then they work together with the police to make the arrest, or engage in a massive firefight with bullets pinging off aluminum light poles and steel railings, and of course, somebody gets shot in the shoulder or maybe just a flesh-wound to the leg. And then the suspect escapes through the underground sewer system and comes up through a manhole under a van where he accomplices are waiting for him. I believe that's the typical scenario.

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Yes, they would go to wherever the suspect is. If they don't smash the door down, I could see them surrounding the building, and ordering everyone out with a loudspeaker. Heavily armed, possibly have an armored vehicle, it depends on the severity.

Auteur said:

So that was a response to an act of terrorism, and the situation in my novel is to prevent an act of violence. So, in this case, the police and FBI would work together on stopping it. But I guess they would work together in the first case, also. The FBI primarily does the investigative work and then they work together with the police to make the arrest, or engage in a massive firefight with bullets pinging off aluminum light poles and steel railings, and of course, somebody gets shot in the shoulder or maybe just a flesh-wound to the leg. And then the suspect escapes through the underground sewer system and comes up through a manhole under a van where he accomplices are waiting for him. I believe that's the typical scenario.

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Or, there's the alternate scenario - the suspect is scuttling through the sewers when he runs into a giant 'gator, who's been waiting a long time for a decent meal. The world is full of endings, some more likely than others.

Be careful with this "tip" concept, especially about what somebody might do. If the contacted LE agency can find no corroborating evidence that a crime has been committed there is likely very little they can actually do. There have been plenty of "tips" that have proven to be bogus (estranged spouses, disgruntled co-workers, etc.) and led to innocent people being subjected to unwarranted scrutiny and worse. The potential for abuse and the (inevitable) subsequent civil liability has LE agencies proceeding most carefully. Does the recent rash of SWATing incidents--an anonymous tip that warrants an emergency SWAT deployment that targets an (innocent) individual--ring a bell?

To make your story plausible have your suspect actually commit a criminal act, something that your "tipster" can report with sufficient details that can be corroborated and thereby rise to the level of probable cause. Mere suspicion won't do it--think actual evidence of a crime.

Auteur said:

Thanks everyone for your input!

I watched a video a few weeks ago about how the FBI apprehended a suspect who planted a bomb near a parade route. It didn't detonate, but it was still treated as an act of terrorism. After they found out who did it, they suckered him into leaving his house and then used construction equipment to stop him on a rural road in eastern Oregon (or it might have been Washington) because there would be less likelihood of a gun battle if the perp was away from his home arsenal. The action occurred back in the '80s, I believe.

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Not really due to any potential home arsenal or worry of gun battle. If they were the prime suspect in an attempted bombing attack, the worry would be explosive devices in or around their home, especially booby traps for 'when the feds came.' A rural rd, with no chance of bystanders, eliminates a lot of those worries and greatly increases the safety of the arresting officers.

As the scenario was presented in the OP, they wouldn't do any of those things. The local police would be much more likely to get involved once he presents an imminent threat. Like posting pictures of all those guns with the message, 'my ex and I are going to have a little chat,' which said ex sees and reports to police that she's in fear of her life. Still probably wouldn't result in the kind of raid mentioned by the op.

Put him in a state with a red flag law. Local cops, not FBI. Unless he's a known terrorist, suspect in a bank robbery or suspected serial killer, it's unlikely FBI would get involved. Before or after.

Jeff