U.S. officials wonder: Did Turkish leader's coup memories drive Russia arms deal?

FILE PHOTO: People walk past Russian S-400 missile air defence systems before the military parade to commemorate the 75th anniversary of the battle of Stalingrad in World War Two, in the city of Volgograd, Russia February 2, 2018. REUTERS/Tatyana Maleyeva/File Photo

WASHINGTON (Reuters) - For some U.S. officials, the Trump administration's failure to persuade Turkey not to buy a Russian air defence system may have its roots in a coup attempt against President Tayyip Erdogan three years ago this week.

While they recognise that worsening U.S. relations with Turkey in recent years and Russia's growing clout with Ankara may also have helped sway Turkey towards buying the S-400 system, three U.S. officials and a defence source told Reuters of another working theory that has gained ground within the Trump administration.