'Have been musing about how the NSA and the GCHQ got so chummy, and this opinion piece also made me wonder about how our own system evolved over time in the way it did. Based on the anecdotal evidence, it seems that the U.S. has always been dependent in some ways on the British scheme of intelligence, as structured in government. We forget nowadays that, in the longer view, the FBI, the CIA, and the NSA are all fairly recent constructs--the FBI is a little more than 75 years old (although it grew out of the earlier BOI--Bureau of Investigations--created about 25 years previously). The CIA is only about 65 years old, and the NSA, a mere stripling at 60 years old.
To a considerable degree, we've followed the British model of organization. The FBI is the analog of MI5, the CIA that of MI6 (thus purportedly separating domestic and foreign intelligence pursuits), with GCHQ originally handling signals intelligence and cryptography for both the military and civilian government, as the NSA has done for much of its existence (the notable exception being that NSA has its mandate as a military operation, with military leadership and funding from military budgets).
We've also followed the British tendency of taking its prime intelligence recruits from elite institutions--Cambridge, Oxford, Eton, Sandhurst, and the like, as our intelligence services are fond of Yale, particularly, and Harvard, and for a time during the Cold War, from elite Catholic universities such as Fordham and Notre Dame, most likely because idealistic young Catholic students might reliably become good Cold Warriors in the fight against godless Communism.