Despite a general agreement between the United States and the United Kingdom that their citizens won't be surveilled by each other's intelligence agencies, documents leaked by Edward Snowden indicate that, since 2007, the National Security Agency has been explicitly collecting and analyzing information on British citizens, with that country's permission. Raising the question: Could British intelligence be surveilling us — with the NSA's permission?

The new report in The Guardian outlines what's known about the agreement between the countries, two of the five nations that comprise the "Five Eyes" group (the other three being New Zealand, Australia, and Canada). It appears that the collection is largely metadata — email addresses, phone numbers, and IP addresses — collected incidentally, meaning without both people involved in the communication being specific targets of the agency. That data is then apparently used in the NSA's system for building out relationship networks.

The NSA has been using the U.K. data to conduct so-called "pattern of life" or "contact-chaining" analyses, under which the agency can look up to three "hops" away from a target of interest – examining the communications of a friend of a friend of a friend.

The Guardian also notes that this — though undertaken with the U.K.'s consent — runs contrary to the understanding between Five Eyes countries.

The memo states that the Five-Eyes agreement "has evolved to include a common understanding that both governments will not target each other's citizens/persons".

But the next sentence – classified as not to be shared with foreign partners – states that governments "reserved the right" to conduct intelligence operations against each other's citizens "when it is in the best interests of each nation".

The United States made unilateral contingency plans to surveil even its allies. "The document does not reveal whether such operations had been authorized in the past," the paper continues, "nor whether the NSA believes its Five-Eyes partners conduct operations against U.S. citizens."

This raises the question raised above. Could British intelligence be collecting and analyzing information on Americans through its own or joint intelligence systems? Could the NSA explicitly use GCHQ (as the agency is known) to get around the search prohibitions outlined under the Fourth Amendment?

The Wire spoke with Electronic Frontier Foundation senior staff attorney Lee Tien to ask him those questions. And the answer is, no.

"It's not that easy to circumvent the Fourth Amendment, assuming all the facts were clear," Tien told us. There exists longstanding case law known as the Joint Venture Doctrine, that implicates the government in any unconstitutional behavior undertaken with its knowledge or participation. "Generally, the idea is that if the U.S. joins with a foreign government in conducting a search then any consequences of that search … the U.S.A. would still legally be on the hook for."

"There's another doctrine, a related one, that talks about circuitous and indirect search methods. Again it's the same kind of idea," Tien told us. "I think there's a fairly reasonable argument that you can't launder your way around the Amendment." Even if the agreement were of the wink-and-nudge variety, "the fundamental question is always going to be: how much was the U.S. involved."

Could the GCHQ be surveilling Americans without telling the United States? It could — as could the intelligence services of other countries. It's not clear if the UK is given a pass on surveilling Americans in the same way that we're okayed to look at Britons. But if it is doing so, and the NSA knows about it or benefits from what Tien calls the "fruit" of that surveillance — the NSA would likely be accountable.

Assuming there were a robust system of accountability in place.