WARNING! THIS IS A SENSITIVE TOPIC TO BE ANSWERED! DRAMA ON COMMENTS SECTION WILL NOT BE TOLERATED!

So I scrolled someone's comments and noticed someone asking a question
Where were interceptors on the September the 11th?

This post is not meant to start a fire in the SP community nor to get my self burned alive by mods but an information about a certain historical day

The answer

There were interceptors during that time

the following is an example

At 08:13, the pilots of Flight 11 responded to an instruction to turn 20 degrees to the right issued from Boston Center. A few seconds later, Boston Center instructed the pilots to ascend in altitude to which there was no response. Multiple attempts to contact the flight went unanswered and as it became apparent that Flight 11 began to pose an air hazard, air traffic controllers began to reroute arriving aircraft for adequate separation.[6] Boston Center flight controller Tom Roberts said "We had pretty much moved all the airplanes from Albany, New York to Syracuse, New York out of the way because that's the track he was going on.'"[6][7] At 08:19, Betty Ong, a flight attendant on Flight 11, called the American Airlines reservation desk to report that the flight was hijacked.[8] At 08:21, Flight 11 stopped transmitting its transponder information concerning its altitude or identification.

At 08:24, the controller heard what he believed was the voice of a hijacker (believed to be Mohamed Atta) in a radio transmission from Flight 11. Two minutes later, the plane veered off course and turned south near Albany, NY. The Boston Center called the FAA Command Center at Herndon at 08:28 to report the hijacking. At 08:32, Herndon called FAA Headquarters in Washington. At 08:34, Boston Center contacted Otis Air National Guard (ANG) base to notify them of the hijacking.[9] The controller at Otis directed Boston to contact NORAD's Northeast Air Defense Sector (NEADS), and then informed the Otis Operations Center to expect a call from NEADS ordering a scramble. At this time two pilots began to suit up and drove to their waiting F-15 fighter jets.[10][11][12] At 08:37, Boston Center contacted NEADS in Rome, New York. This was the first report of a hijacking that reached NORAD.[2]

Two F-15 alert aircraft at Otis Air National Guard Base in Falmouth, Massachusetts were ordered to battle stations (seated in their aircraft, engines not yet started). At 08:46, just as Flight 11 struck the North Tower of the World Trade Center, the two F-15s were ordered to scramble (an order that begins with engine start-up, a process that takes about five minutes), and radar confirmed they were airborne by 08:53.[12] By that time, however, the World Trade Center's North Tower had already been hit.[13][14][15][16][17][18][19]

At that time, NEADS personnel were still trying to pinpoint the location of Flight 11, but since the plane had already struck the North Tower, they were unable to. Without having a specific target located, military commanders were uncertain where to send the fighters.[4] Boston Center controllers were still tracking Flight 11 as a primary target but were unable to communicate its location to NEADS by phone.[3]

After news of an aircraft hitting the World Trade Center began spreading, no decision was made to alter the course of the F-15s of the 102nd Fighter Wing.[4][10] A decision was made to send the Otis fighters south of Long Island rather than straight to New York City, as originally ordered by Maj. Nasypany of NEADS.[4]

One of the two pilots, Lieutenant Colonel Timothy “Duff”[20] Duffy, would later state he had already heard about the suspected hijacking (attributed to a phone call from the FAA's Boston Center) as he was supervising training exercises at Otis ANG base.[12] Claiming to have a "bad feeling about the suspected hijacking", he and his wingman, Major Daniel “Nasty”[20] Nash, decided to use their F-15s' afterburners.[10]

Flying supersonically, the F-15s were just south of Long Island when United Airlines Flight 175 smashed into the World Trade Center's south tower. NEADS wanted to direct the fighters over Manhattan, but FAA air controllers, fearing collisions with civilian aircraft, told NEADS to hold off. According to the FAA, there is an average of 200 flights per 24 hours over the Hudson River in the vicinity of NYC.[21] The fighters were then ordered in a holding pattern off the coast of Long Island (in military-controlled airspace), where they remained from 09:09 to 09:13. After the airspace was cleared, the Otis fighters were directed towards Manhattan, where they arrived at 09:25 and established a combat air patrol (CAP).[2][4][22]

The 9/11 Commission report stated the Boston center tried to contact a former alert site in New Jersey, but it had been phased out. The report stated there were only seven "alert" sites (sites which had a pair of aircraft armed and on hand) in the United States by 9/11 and just two in the Northeast Air Defense Sector—Otis Air National Guard base in Massachusetts and Langley Air Force Base in Virginia.[23]
sauce: Wikipedia

Where there F-16s in the same day?

There was. Two F-16s were scrambled to attempt to take down UA Flight 93 by ramming it but failed so the F-16s went out for a patrol instead and escorted AF 1 to Washington iirc.

Question: What happened post 9/11?

After the 9/11 attacks. OP: Noble Eagle was commenced to stop events from happening again.

Additional Notes: Sauces are from Wikipedia