There's going to be a colossal manhunt. Every possible technology will be mobilized. And it's very hard not to slip up on opsec. Unless the guy leaves the country very quickly, I would expect him to be caught (or killed resisting arrest, the common fate of mass shootings).

When I was in college a kid used a computer in a lab to send a death thread against Bill Clinton to whotehouse.gov. I recall this in part because it was the lab I did most of my hours in both as an employee and because it was near my friend’s appt so we would study there. Dude sent it from a computer two rows back from where I usually sat.

Someone got up to use the bathroom and didn’t lock the machine. Dude thought he was being funny. But of course since he logged on to the adjacent machine he put himself on the suspect list and got caught. And in a hell of lot of trouble as I recall. I think he got expelled, too.

That was for a prank, not an assassination.

The thing some crime dramas don’t get right is that while circumstantial and tainted evidence cannot get you a conviction, it is absolutely possible for it to be used to prioritize manpower used to narrow you down to the top of the list.

There’s a thing in law enforcement called Parallel Construction. It can be used to protect confidential informants such as in undercover operations, but it can also be used to replace evidence that was found illegally, such as illegal recording or theft by a neighbor.

They just need to find something that follows process front to back. They don’t need to do that in order to figure out it was you in the first place.

Statistics say the spouse or partner almost always committed the murder. Even lacking any evidence they look really really hard at these people. It’s not illegal or unfair to do so. It’s triage. If I’m looking at Mrs Fredrickson’s murder, I’m not looking at any cold cases or spending effort on many other active cases. It’s unfortunately a numbers game.

Note: it was an assassination, not a mass shooting. There was only one shot.

very likely he will be caught by his friends or family- everyone that does something like this slips up. The guy that shot United Healthcare's CEO was outed partially by his own mom in fact.

Depends on shooter's background. For state actors the easiest way to undermine US is to continue pushing towards more political violence in US via any and all means.