Congress has absolute authority to summon anyone in from anywhere in the world and grill them.
If it is secret stuff, then close hearing.
If a majority of commitee does not so vote, then they can have the rump hearings that Conyers and Waxman always pulled.
As more people become aware of allegations that Brennan was runing secret ops in Libya, elsewhere, to stir things up, there should be more pressure to do so. So far Congress only seems willing to question the folks with the most to hide/gain from any cover-up, vs. getting to the whole truth of what happened.