"The reduction seen in US government debt in the late 1990s has led to a redution in the supply of intermediate and long-term government bonds, and some concern has arisen over this fact. In the United States, some efforts have been made to promote the long-term debt of Fannie Mae and Freddie Mac as substitute benchmark bonds."
This comes from a passage connected to US treasury bond futures.
I cannot understand why Freddie Mac and Fannie Mae have been chosen as closest to government debt? Didnt one of them go bust in the recession making them not quite default free (as one usually assumes in US debt)?