US treasuries mostly in late march. Demand for dollars skyrocketed due to covid-19 and people were selling their treasuries to get dollars causing treasury yields to spike so the FED started buying $75 billion in Treasury securities per day to supply liquidity to the Treasury market and ended up buying over $1 trillion in Treasury securities within a short 3-week window.

More analysis about it at https://www.lynalden.com/august-2020-newsletter/