Reply #28: So this controller

[Corgigal]

from this article written Sept 13, 2001 must have been mistaken.

Flight 11's transponder had stopped working. It was no longer sending a radar pulse. The plane's altitude also became a matter of guesswork for controllers, though the Boeing 767 was still visible on radar. Still, the controllers hoped that the plane simply had an electrical problem

link: http://billstclair.com/911timeline/2001/csmonitor091301.html