First, there's no way agencies would have shared information as one of the most glaring mistakes gleaned from the 9/11 Commission's report is that bureaucracy prevented such agency cooperation.
Lastly, King presupposes that 9/11 was solely a terror problem. As I've addressed previously, 9/11 began as an immigration problem:
You didn’t need NSA or its surveillance system to thwart the 9/11 hijackers. Most were in the United States on expired visas hiding in plain sight. Mohammed Atta was pulled over for a traffic violation and never showed in court:
On August 23, Atta’s driver license was revoked in absentia after he failed to show up in traffic court to answer the earlier citation for driving without a license. On the same day, Israeli Mossad reportedly gave his name to the CIA as part of a list of 19 names they said were planning an attack in the near future.
We had systems in place to prevent this terror attack and they failed. Government didn’t need to mine metadata for this. Government simply needed to enforce our immigration laws. Atta and the others weren’t hiding — even after we were reportedly given from Mossad that one or more of these individuals were planning a future attack. Who knows how many more may be prevented as well, if only we were able to properly manage immigration by securing our borders and more selectively issuing visas.
If PRISM was so successful, why didn't it discover Nidal Hasan, who publicly discussed extremism on the Internet? Why didn't it discover his email exchanges with Anwar al-Awlaki? Why didn't they discover the Tsarnaev brothers (and Russia gave us intel)? The Times Square Bomber? These were the easiest, out in broad daylight, red flags galore. King's justification of violating Americans's Fourth Amendment civil liberties via fear mongering is offensive.