[aarontait]

The title is somewhat misleading. A groundstop is far less dramatic than "grounding" all flights (including ones already en-route). Grounding all flights would be akin to what the FAA ordered during the 9/11 attacks.

[dmitrygr]

Multiple planes were told to land immediately. I personally know two pilots who were told to do so.

[aarontait]

Interesting. Were they operating commercial passenger flights? The FAA's statement only mentions a groundstop.