John Walton was Minardi's F1 Sporting Director and because this happened the Minardi had no sponsors on its body if you noticed and it said John Walton on various places (bottom post)
I translated this from dutch for you:
(09 July 2004) Minardi’s Sporting Director John Walton was this afternoon struck by a heart-attack. In critical, yet not too bad condition he was checked into the hospital. Team manager Paul Stoddart had said that John Walton was't feeling good earlier that week, on Thursday evening he was in very bad and poor condition. It may be clear therefore that we are all deeply shocked with this accuring. Our thought our therefore with his family all weekend.