From the news report I saw, if I remember correctly, the police showed up after the 911 call by Travis McMichaels (on February 11) and Mr. English was being appraised of the situation by one of the neighbors.
You might want to look for this more recent news. I'll try to get back to you if I find I misremembered that.
If you missed other news before, Mr. English had contacted the police on two occasions himself about trespassers on the property.
But I think Babbylonian has a good point. If the basis for the citizen’s arrest was trespassing, the McMichaels were basing it on second hand information. A neighbor told them that the homeowner told him that he didn’t want the guy on the property.
And it was two weeks ago. They had no idea what might have transpired in those two weeks. English wasn’t talking to them.
To throw out a complete hypothetical: Maybe a couple days after 2/11 English gets a call from his brother. He tells him his good friend is very interested in his construction project and he told him he should stop by when he has a chance and see how it is developing. English realizes that the guy caught on the video tape isn’t some burglar but is his brother’s friend. He’s perfectly fine with him going onto the property. Maybe he even tells his neighbor what is going on. Maybe he even tells the police that this isn’t a problem. The McMichaels know nothing about this.
I’m certainly not saying that is what happened. The point is, when McMichaels see the guy running down the street, they don’t really know whether or not the guy was actually committing a crime. They don’t actually know whether or not he was currently authorized to be on the property.
If Arbery had been detained and not killed, I think the police would have ended up arresting him. But I think they would have had to contact English to verify that he was not authorized to be on the property to maintain that arrest, and maybe even before the arrest.
It could not be actually known whether or not Arbery was authorized to be on the property until the property owner confirmed that fact. Police have the power to detain and question and investigate. Private persons do not. Without some investigation, verification of the crime could not be confirmed. That would mean the McMichaels could not have known whether or not a crime had actually been committed, and therefore could not make a citizen’s arrest.
(This gets a bit more complicated when we get to the second part of the citizen’s arrest, which I will try to do a little later.)