Wall Street closed in firmly negative territory on Monday, continuing a rout that's reaching historic dimensions. After Trump's disruptive tariff policies, it's now his attacks on the US Federal Reserve that are unsettling investors. The US president has been lambasting the head of the central bank, Jerome Powell, on social media. He called Powell "a major loser" and urged "pre-emptive cuts in interest rates". We take a look at the Federal Reserve's mandate and why it's supposed to be independent of political pressure.