It was a lot worse than that. This would’ve been a credible threat that various intelligence agencies in the US were aware of.

Al-Queda was already involved in other terrorist attacks and was known to US intelligence.

All the different agencies had info on a lot of the hijackers but they all kept it to themselves or the info wasn’t passed up the ladder.

The Looming Tower by Lawrence Wright is one of the best books on this topic and goes further into it. But I’m sure you can find other references, including the 9/11 Report.

This was a massive failure by the US intelligence community.

---- Edit:

I'll add some references so my comment is a little more contributory besides just "read this book."

https://oig.justice.gov/sites/default/files/archive/special/...

This special report goes into it. Here is a quote from the report:

"The intelligence led to a surveillance operation in Malaysia in which it was discovered that Mihdhar had a valid multiple-entry U.S. visa and photographs of Mihdhar meeting with other al Qaeda operatives were taken."

The CIA had this information and should've passed it on to the FBI. (CIA operates outside the US, FBI operates inside the US.)

The report continues:

"We reviewed whether this information was in fact passed to the FBI by the CIA, and based on the evidence, concluded that while the CIA passed some of the information about Mihdhar to the FBI, it did not contemporaneously pass the information about Mihdhar’s U.S. visa to the FBI. We concluded it was not disclosed by the CIA until late August 2001, shortly before the September 11 terrorist attacks. We also reviewed whether FBI detailees to the CIA contemporaneously acquired this information and what action, if any, they took with respect to this information."

Then there's the FAA-related stuff, which is what brought on the TSA. This is from the 9/11 Report:

"The next aviation security layer was passenger prescreening. The FAA
directed air carriers not to fly individuals known to pose a “direct” threat to
civil aviation. But as of 9/11, the FAA’s “no-fly” list contained the names of
just 12 terrorist suspects (including 9/11 mastermind Khalid Sheikh
Mohammed), even though government watchlists contained the names of
many thousands of known and suspected terrorists."

A few paragraphs down, the report explains why the known hijackers were still allowed to fly:

"Although the algorithm included hijacker profile data, at that time only pas-
sengers checking bags were eligible to be selected by CAPPS for additional
scrutiny. Selection entailed only having one’s checked baggage screened for
explosives or held off the airplane until one had boarded. Primarily because of
concern regarding potential discrimination and the impact on passenger
throughput, “selectees” were no longer required to undergo extraordinary
screening of their carry-on baggage as had been the case before the system was
computerized in 1997. This policy change also reflected the perception that
nonsuicide sabotage was the primary threat to civil aviation."

This is just two examples I wanted to quickly dig up. There's an entire list of failures you can find if you read the reports or the book I mentioned earlier. It's quite good reading, I promise! Understanding the failures also gets you to understand the changes that came after 9/11. This includes things like the PATRIOT act, TSA, Department of Homeland Security (DHS), etc. DHS is meant to force the intelligence communities to share intel with each other rather than "compete" for it.