Text updated 12 September 2018:
In December 1997 I gave a very specific 9/11 style attack warning whilst working at a print factory in Brooklyn. I specifically asked several people to pass my urgent and desperate warnings to authorities.

They asked why I could not report it myself - I explained I was a in a foreign country & approaching any security agency with such a threat could have dire consequences for me if misunderstood. I said I did not want to risk my job and that I was scared I’d be misunderstood. 

That’s when they decided to call over Mr. Barkochba, who came in from another room a few hours later. 

The machine operators told me something like, "This is the man to tell"

I repeated my warning in detail to Mr. Barkochba and asked if he knew any security services. He responded that he did know ‘some important people in that field’ and that he would tell them. A vague but authoritative answer. That was when I felt I’d done everything I could have done to have passed on my warnings and fears. 

I’d warned the airline, I’d passed on a warning to security service in Canada and now in New York I had warned a man who intimated he had direct connections to the secret services. 

The next morning when visiting the Empire State I did consider walking to the WTC, but again a fear of being misunderstood (making a threaten rather than giving a warning) prevented me. I had a good job, travelling the world and I did not want to risk that. I gave what I thought were sufficient warnings, but I was wrong. I am sorry.

Note added July 2019

I gave my warning to a US citizen because I did not know who to contact and feared being misunderstood, i.e. that I was making a threat rather than giving a warning. I did give Mr Barkochba my business card and I did ask him to identify me to the security services as the origin of this warning.