Brad Garrett

You got detectives at the scene that are immediately trying to figure out what happened. At the same time, you got somebody pulling cameras.

They seized 26,000 ghost guns at crime scenes last year. And the number keeps growing and growing and growing because if you are a felon or somebody that is not allowed to buy firearms, where are you gonna go? We have it made. This shooter knew that if he used a 3D printer gun, they wouldn't be able to trace him.

Think of the word parasite. That's very dehumanizing to start with. Because to kill other human beings, you have to dehumanize them before you kill them.

My name is Brad Garrett. I'm a retired FBI agent. I've investigated single homicides, serial homicides, and mass casualty homicides in the United States and in various parts of the world for a better part of 20 years. So the key is, as much information as you can gather about the victim may lead you toward a potential suspect. Who is this guy? Where was he going?

Why was he shot in that location, that time of the day, et cetera? The layer beyond that is who had issues with Mr. Thompson.

He comes in, and he basically works his way through the park.

So this video obtained by ABC News is of a suspect on, presumably, it looks like his e-bike, riding away from Central Park. This is important because you always want to know direction of flight because it'll give you leads. Since you don't know who he is, it'll give you directions about where he possibly is going.

What's interesting about this case is we went on for multiple days of a lot of pictures of this suspect in various locations, and nobody came forward to ID him.

With more on this, I'm joined by former FBI agent Brad Garrett. He joins us today from Washington. Brad, good morning. Good to see you. So autopsies have now revealed the cause of death to be ligature neck compression, strangulation. Homicide detectives have taken over. What does this tell us about what might have happened? What are investigators looking for?

And so what would make you think then that it's someone that would know the couple?

And given the effort, I'll call it, that was put into their deaths, do you think that one person could have done this on their own, or would there be more people involved?