In theory the FBI is supposed to be responsible for domestic security, the CIA for international. Wikileaks is about as international as you could want and therefore shouldn't really be in the FBI's ballpark.
That's wrong. The GP is right. The FBI is responsible for investigating any leak of US secrets. They will investigate if the CIA leaks secrets, the military leaks secrets, or Russian agents steal American secrets.
People hear CIA and thoughts of horrible conspiracy theories overwhelm. But it is in abstract principle the more appropriate organization to have gotten involved.
It's not hard when they kidnap people, take them to far away countries and torture them. If any non-US allies behaved like this we would never hear the end of it.