NEW YORK (Reuters) – The brief crash in Nordic markets on Monday was due to a single sell order placed by Citigroup, according to two sources familiar with the matter. The flash-crash, which caused European shares to suddenly fall on a day of holiday-thinned trading activity, involved an erroneous calculation relating to a Nasdaq index involving Swedish companies, one source said. Citi declined to comment. (Reporting by John McCrank; Editing by Chris Reese)