Comments so far have already said it. Wait for LoO.

But I'll give you an example...

I have a few coworkers/former coworkers in similar situations recently. Security clearance in process and legitimately the only item holding up the LoO since late last year. They were in contact with the hiring manager and they reassured them that they really need them in the position and the target date was still as agreed upon (2 weeks after security clearance has been processed).

Budgets came in for the fiscal year and upper management shutdown hiring for those positions. The teams really needed them for the work they would be doing (and still do) but it was not a priority to the departments/branches budget-wise.

Bad luck really, because had their clearances came in a lot sooner, they would be in those positions months ago, before the positions were even in consideration to be shutdown.

Not to say all verbal offers fall apart, I was in the exact same situation last year and once my security clearance came through, the letter of offer followed very shortly. It just goes to show you, to reiterate what others have said, until you have the letter of offer in hand and have signed to accept, it is not certain. There are factors that you don't know and even the manager that wants to hire you doesn't know about behind the scenes that can come up and as a result, the verbal offers fall through.