the world, paid nothing, not a dime in federal income taxes that year. same story for tesla billionaire elon musk in 2018. investment icon warren buffett says he supports higher taxes for the rich. >> the wealthy are definitely undertaxed. >> reporter: and he, too, has benefited, between 2014 and 2018, buffett made $24.3 billion. but over those years, he paid just $23.7 million in federal income taxes. >> families have to pay their tax bills regularly but the billionaires have found a way to avoid them. >> reporter: pro publica found nothing illegal. its investigation focused on federal income taxes, which are largely based on wages and salaries. but these billionaires make most of their money from investments, which have long been given preferential treatment, not from salaries. and that skewers the federal tax code towards the wealthy.