One of the biggest threats to US security may now come from within Europe, US Homeland Security head, Michael Chertoff, has said.He said militant attacks and plots in Europe over recent years had made the US aware of the "real risk that Europe will become a platform for terrorists".He further cited deadly bomb attacks on Madrid and London, and a terror alert affecting UK-US flights in August 2006, as well as "people travelling from South Asia and the Middle East into Europe and carrying out attacks there".Mr Chertoff then said it was likely security checks on travellers from particularly Europe would from now on be increased.SOURCE: BBC