If it were me, I would probably call in professionals to get a thorough inspection and make sure all the squirrels were out and any possible entry points were addressed. The other benefit of pros is that you’ll usually get a warranty period where they have to come back (at no charge) if the problem wasn’t solved.

If you are set on DIY, the best bet would probably be trapping and removing them yourself. Live traps go for about $30. You would however need to access that attic to set up and check the trap(s).