In any murder mystery film, it pays to watch the boring grey man (or woman) in the corner; quiet, unobtrusive characters can be deadly.

So, too, in finance. Four years ago, the giant US money market funds seemed some of the dullest actors in the global financial scene. But in 2007, they quietly helped to spark the crisis in the mortgage-backed securities world, when they silently stopped rolling over bonds. Then, in 2008, they furtively wielded the knife again, pulling funding from some American banks and the “repo” – repurchase – markets.

Now, their shadow looms again. As my colleagues Dan McCrum, Telis Demos and Jennifer Hughes have reported, in recent weeks these funds have been quietly backing away from European banks, either refusing to roll over loans, or slashing the maturities of the funds they provide. Fitch, for example, recently calculated that the largest US money market funds cut their exposure in absolute terms by $30bn in July, even before the latest turmoil.