A credible threat from Iran? More likely a technical issue.

Happened in 2023 https://www.faa.gov/newsroom/faa-notam-statement

Jan 2023 was the most recent nationwide one.

https://www.cnbc.com/2023/01/11/faa-orders-airlines-to-pause...

https://en.wikipedia.org/wiki/2023_FAA_system_outage

previous time it happened right before chinese baloon made the news

it may be something similar - remove all random, but controllable sources of radar return for military to start scanning

It happened on 9/11, didn’t it?