Without either the leaker or the journalist it was leaked to admitting who was responsible (both unlikely to do so) it could be impossible to prove. Obviously the contents of the letter were discussed prior to the letter actually being leaked, as the Intercept reported about it on the 12th of September, but the actual letter doesn't seem to have been leaked until just prior to the New Yorker's story on the 14th. Both the content of the letter and the letter itself could have been leaked by multiple sources. The contents could have been leaked by Echoos or someone in Echoos' office, Feinstein or someone in her office, anyone Ford told about it, Ford herself, someone within her lawyer's office or - depending upon when Feinstein turned the letter over to the FBI - someone within that agency. The actual letter could have been leaked by any of the above listed or someone in the WH or any of 100 Senators or someone within any of their offices, as all had access to the letter by sometime on the 13th. Ford herself admittedly contacted WaPo's tip line in early July prior to the letter even being written, so it's really not a big issue to me and hardly worthy of an expensive investigation, as the story was intended to come out., and it would have come out...one way or another.