Transponders off and someone playing with them remotely:

Between 8:13-8:21 a.m.): Flight 11 Transponder Turned Off Shortly after flight controllers ask Flight 11 to climb to 35,000 feet, the transponder stops transmitting

8:46 a.m.: Flight 175 Changes Transponder Signal but Remains Easily Traceable

Flight 175 stops transmitting its transponder signal. It is 50 miles north of New York City, headed toward Baltimore. [9/11 Commission Report, 6/17/04; Newsday, 9/10/02; Guardian, 10/17/01] However, the transponder is turned off for only about 30 seconds,

and then changed to a signal that is not designated for any plane on that day. [Newsday, 9/10/02

(8:56 a.m.): Flight 77 Transponder Signal Disappears; NORAD Not Informed

(9:40 a.m.): Flight 93 Transponder Signal Turned Off; Flight Still Closely Tracked