The officer at the US embassy informed me that my authorisation to travel had been revoked because the ‘algorithm’ had identified a security threat. He said he did not know what had triggered the algorithm but suggested that it could be something I was involved in, people I am or was in contact with, places to which I had travelled (had I recently been in Syria, Iran, Iraq, Yemen, or Somalia or met their nationals?), hotels at which I stayed, or a certain pattern of relations among these things. I was asked to supply the embassy with additional information, including 15 years of travel history, in particular where I had gone and who had paid for it. The officer said that Homeland Security investigators could assess my case more promptly if I supplied the names of anyone in my network whom I believed might have triggered the algorithm. I declined to provide this information.