After 2001 the demand for liquid safe assets increased so much that the supply has been insufficient; the demand of such low risk-low return securities has been driven by oil producers, exporters countries and wealth managers that look for short term securities at very low risk (INTERNATIONAL CASH POOL - ICP).
The globalisation of financial markets and the portfolio diversification have increased the demand for safe assets, i.e. assets guaranteed by sound sovereign states with positive return, all tough not high, in portfolio of global financial players, like hedge funds, sovereign wealth funds, pension funds, institutional investors and assets managers.
The safest asset in this category is considered to be the US TREASURY BILL, together with the German BUND. The supply of Treasury Bills has not increased in the last decade at the same pace of the demand for safe assets. The gap between demand and supply has been filled by shadow banking securities, such as repo, assets backed commercial papers, and other short term DERIVATIVE(credit related).
Pozsar Zoltan (2011) Institutional Cash Pools and the Triffin Dilemma of the U.S. Banking System, IMF Working Paper n. 190, August.