U.S. stock index futures pared losses early Thursday after the European Central Bank announcement it will buy 750 billion euros' worth of securities and bonds through the end of 2020 to offset the economic impact of the COVID-19 pandemic. Futures have zigzagged. After initially falling sharply at the start of the overnight session, Dow Jones Industrial Average futures surged more than 300 points, or 1.5%, after the ECB's announcement, but then started to freefall. However, Dow futures YM00, +0.01% were last down just 179 points, or 1%. S&P 500 futures ES00, +0.03% and Nasdaq-100 futures NQ00, +0.10% were down 1% each. Earlier in the day, U.S. stocks finished sharply lower, with the Dow DJIA, -0.87% closing below 20,000, its worst mark since 2017. President Donald Trump on Wednesday evening signed a coronavirus bill providing paid leave and free testing, as the administration and Congress turned its focus to broader measures that could stimulate the U.S. economy and help workers.