Why Weren’t the 9/11 Hijackers Stopped at Airport Security?
The 19 hijackers were not stopped because 2001 airport security was built to catch a smuggled bomb, not a passenger willing to die in the act. Blades under four inches and box cutters were legal to carry on planes. Ten hijackers were flagged by a screening system, and being flagged changed nothing about whether they could board.
By the Conspiracy Podcast team | Last updated: August 2026
Twenty five years later, the debate is not about whether the hijackers got through, everyone agrees they did. It is about how close the system came to working anyway, and how one officer’s gut call in Orlando may have changed United 93’s outcome. Here are the questions people actually search about that morning.
Why weren’t the 9/11 hijackers stopped at airport security?
They were not stopped because nothing they carried was illegal and nothing they did triggered an intervention. Airport screening in September 2001 was run by private contractors under rules written after the 1988 Lockerbie bombing, aimed at bombs in checked luggage. Knives under four inches, box cutters, and even mace were allowed through checkpoints without incident.
The 19 men checked in that Tuesday morning at four airports: a small regional field in Portland, Maine, Boston’s Logan, Washington Dulles, and Newark. Each used its own private security contractor, with no shared watchlist between them; Logan’s checkpoint was run by Globe Security, Dulles by Argenbright. A United ticket agent later told investigators that a couple of the men seemed unable to answer basic screening questions, almost as if they had never flown before.
Security footage that survives from Dulles shows two of the American 77 hijackers setting off metal detectors and getting pulled aside for hand-wanding, which cleared them anyway. On every flight, the hijackers took seats clustered in first or business class, closest to the cockpit, a pattern the 9/11 Commission later flagged as deliberate. Mohamed Atta’s own checked bags missed his Boston connection and were recovered after the attacks, containing a handwritten document of spiritual preparation investigators call the last night.
What is the CAPPS system, and how did it fail on 9/11?
CAPPS, the Computer Assisted Passenger Prescreening System, was the airline’s 2001 tool for flagging passengers for extra scrutiny. It flagged 10 of the 19 hijackers that morning. A flag only meant a passenger’s checked bags were held until the airline confirmed they had personally boarded, a rule aimed at bombers, not hijackers willing to die.
The system existed because of the 1988 Lockerbie bombing, where a bomb was smuggled aboard inside checked luggage by a bomber who never got on the plane himself. CAPPS was built to counter exactly that: if a flagged passenger’s bag made it onto the aircraft without the passenger, security held it. Nobody who designed the rule had imagined the passenger himself becoming the weapon, so a flagged hijacker who boarded normally triggered nothing.
The pattern varied flight by flight. Three of five hijackers on American 11 were flagged, along with three of five on American 77. None of the five on United 175 tripped CAPPS at all, and on United 93 only one of four was flagged. One hijacker, Nawaf al-Hazmi, was separately marked suspicious by a United agent for having no photo ID, not by the computer, and boarded anyway. The hosts walk through that breakdown in the 9/11: Unanswered episode covering the day itself.
Were any of the 9/11 hijackers flagged before boarding?
Yes. Ten of the 19 hijackers, more than half, were flagged for extra screening that morning, either by CAPPS or by an alert human agent. Being flagged never stopped anyone from boarding. It only affected how checked luggage was handled, so a system that caught over half the hijackers still let all 19 fly.
Mohamed Atta was flagged the moment he checked two bags at the small Portland, Maine airport at 5:41 a.m., before a short commuter hop to Boston. The 9/11 Commission never determined why he took that roundabout route instead of flying straight out of Boston, though the hosts speculate it was meant to look ordinary. Being flagged meant his bags were held for confirmation and nothing more.
The hosts note that a 55 percent flag rate is strange for a system usually described as near-random, and they raise, half joking, what caused so many of the same cell to be picked. That is their own speculation on air, not a conclusion in the 9/11 Commission Report, which does not name a specific trigger beyond CAPPS’s usual profiling factors like one-way tickets and cash payments.
Could 9/11 have been prevented at the airport?
There is no way to know for certain, and the record does not support a clean yes. What the record does show is that the system worked exactly as designed and still failed, because it was built to stop a smuggled bomb, not a hijacker, and no single missed flag would have grounded any of the four flights.
The single moment historians point to as genuine prevention happened six weeks earlier and states away, not at any of the four boarding gates. An immigration inspector’s decision to deny entry to a suspected fifth hijacker in Orlando kept United 93’s crew at four men instead of five, which the hosts call the closest the country came to altering that day’s outcome.
Some of the failure was structural. According to the 9/11 Commission Report, the CIA had identified two of the hijackers, Khalid al-Mihdhar and Nawaf al-Hazmi, as al-Qaeda operatives over a year earlier, but their names were not added to the State Department’s watchlist until August 23, 2001, weeks after they were already living in the country. No gate agent could flag a name that had not reached the list.
| Theory | What the record shows |
|---|---|
| The system worked as designed. CAPPS flagged 10 of 19 hijackers. | A flag only held checked baggage. The rule targeted a Lockerbie-style bomb, not a hijacker planning to stay aboard. |
| Screeners missed the signs. Agents noticed red flags, like a missing photo ID. | Private contractors like Globe Security and Argenbright logged it, but no screener could deny boarding on suspicion alone. |
| Intelligence agencies sat on what they knew. The CIA tracked two hijackers for over a year. | Per the 9/11 Commission Report, their names reached the watchlist only on August 23, 2001, weeks before the attack. |
| One person actually changed the outcome. An inspector’s call in Orlando stands apart from the rest. | Jose Melendez-Perez denied entry to a suspected fifth hijacker on August 4, 2001, leaving United 93 with four hijackers. |
The screening failure sits alongside plenty of unresolved questions from that day, which the hosts pick apart across the rest of the series. Our roundup of famous conspiracy theories covers a lot more of it.
What weapons did the hijackers carry onto the planes?
The hijackers carried legal items: small knives under four inches and box cutters, both permitted on planes in 2001, plus what several accounts describe as mace or pepper spray. On at least one flight, American 77, investigators specifically noted the weapons used were box cutters. Several hijackers also claimed to have a bomb, though no explosive was ever found.
On American 11, the hijackers seated in first class moved first, using knives and what flight attendant Betty Ong described on a 25-minute call as mace, to force passengers toward the back and clear the area around the cockpit. A passenger seated near one hijacker, believed to be the first person killed that morning, was reportedly stabbed from behind. On American 77, the crew that hit the Pentagon is the one the Commission ties to box cutters, not knives.
None of it was smuggled; every item that made it onto those four planes was, by the letter of 2001 rules, fully allowed. That loophole is the direct reason air travel changed so fast afterward. Congress created the Transportation Security Administration that November, and within a few years small blades, liquids, and gels were banned outright from carry-on bags.
Who was Jose Melendez-Perez, and how did he stop a 20th hijacker from entering the country?
Jose Melendez-Perez was a U.S. immigration inspector at Orlando International Airport. On August 4, 2001, he pulled a Saudi traveler named Mohamed al-Qahtani aside, found his story evasive and changing over 90 minutes, and denied him entry under expedited removal. The 9/11 Commission concluded al-Qahtani was almost certainly meant to be the missing fifth hijacker on United 93.
Al-Qahtani had flown in from London on a one-way ticket with about $2,800 in cash, no return flight and no hotel reservation, a profile that draws immigration scrutiny. He was combative from the start, refused to answer questions under oath, and changed his account of why he was traveling every few minutes. Surveillance footage pulled later showed a man waiting in the parking lot to pick him up: Mohamed Atta, the lead hijacker who piloted American 11 into the North Tower.
Because al-Qahtani never made it past Orlando, United 93 left the gate with four hijackers instead of the five-man teams on the other flights. The hosts treat that missing man, combined with the flight’s 25-minute delay, as one likely reason its 37 passengers were able to organize and storm the cockpit before it reached Washington. That story is covered in Part One of the 9/11: Unanswered series.
One immigration officer on one unremarkable Saturday acting purely on instinct is actually what I consider the entire reason Flight 93 ended up with four hijackers that day.
the guys, on the record
