When they put stuff like this together, they do Not just arbitrarily chose a 16 second gap. There's some sort of basis for using the "16" seconds.
With respect to JFK assassination plots, the Chicago, and Florida planned assassinations are documented. And those involved a tall building.
Documented how? By a convicted Secret Service agent who claimed there was an assassination plot in Chicago. Or the tape that used actors to voice a supposed undercover recording detailing a planned assassination of JFK in Miami?
Great sources you have there.
Threats against US presidents are not uncommon and while they are taken seriously by the Secret Service, most of them turn out to be crackpots and not credible threats.