In January 2016, $400m (£290m) was flown by the United States to Tehran in the dead of night. Loaded on to wooden pallets on an unmarked plane, it was the first in a series of instalments to satisfy an unfulfilled American-Iranian arms deal signed in 1979, before the Shah was replaced in the revolution. On the morning after the payment, four American prisoners were released, boarding planes back to their homeland.
The White House insisted the payment and the release were coincidental. But General Mohammad Reza Naghdi, a commander in the Islamic Revolutionary Guard Corps (IRGC), took to Iranian state media to proclaim:
'Taking this much money back was in return for the release of the American spies.'
Five years on, some high-ranking British politicians are lining up to suggest we should do something similar in order to secure Nazanin Zaghari-Ratcliffe’s release.