I agree that speech definately scared some people and then when he pulled off the anti nuclear test treaty , the plot probably escalated .

An almost identical plot on Chicago was spoiled in the last minute when jfk canceled his trip . A tipster named " lee " tipped off the plot that resulted on 2 of 4 Cubans being arrested . This was all hushed up but is common knowledge now , The poor secret service man that blew the whistle the had locked up . A chicago hit would of coincided with the CIA led hit on the Neim brothers / leaders in South Vietnam .

There probably aren't any smoking guns left but if you read the AARB declassified files , it's pretty clear this was a high end military / intelligence hit . Per Richard Nagal the more moderate intelligence men were trying to stop the hit but ultimately failed .