It appears that the Americans are refusing to allow Italian officials to examine the car in which Giuliana Sgrena and Nicola Calipari were travelling.

See:
http://www.armytimes.com/story.php?f=1-292925-740304.php
http://www.aljazeera.com/cgi-bin/news_service/middle_east_full_story.asp?service_id=7490.