Question for the "Oswald did it crowd"

Gil Jesus wrote:I am aware of four people who warned JFK not to go to Dallas.

1. Dallas resident Anne Brinkley ( wife of NBC newsman David Brinkley ) wrote a letter to Pierre Salinger, Kennedy's Press Secretary begging him not to let the President come to Dallas or someone was going to kill him.

2. Senator J. William Fulbright of Arkansas told Kennedy that Dallas was a dangerous place. "I wouldn't go there", he told JFK, " don't YOU go there."

3. Dallas resident Stanley Marcus of the Neimann-Marcus chain felt so strongly about it that he flew to Washington for the sole purpose of warning JFK not to go to Dallas.

4. UN Ambassador Adlai Stevenson had been attacked by a mob just a month earlier and warned the President to stay away from Dallas.

So my question is this :

How did all of these people know that Oswald was going to kill Kennedy ?

I'm sure you're talking tongue-in-cheek here, but to point out the obvious, they were worried about the General Walker crowd and similar groups. 

It seems to have been a pretty strong whisper in the wind throughout Dallas. The only people oblivious to the danger were the police, the FBI and the Secret Service because 99% of the security precautions were aimed at stopping another Stevenson incident with protesters. And that was not for JFK's sake. It was to save themselves embarrassment.

Gil, this has happened since the beginning of time. All politicians have been warned not to go to such and such place. It doesn't mean anything would happen and most likely nothing did happen. There have also been crackpots galore as well.

UPDATE - I found this on Pat Speer's site. If you didn't know it, you'd find these exact same signs at today's rallies - 60 years on - filled with deplorables. 

I think this may have been a little more than a concern for a few folks holding some signs.

Pretty scary stuff.

https://newrepublic.com/article/115601/jfk-dallas-right-wingers-attack-adlai-stevenson