The 500 largest American companies hold more than $2.1 trillion in accumulated profits offshore to avoid U.S. taxes and would collectively owe an estimated $620 billion in U.S. taxes if they repatriated the funds, according to a study released on Tuesday. The study, by two left-leaning non-profit groups, found that nearly three-quarters of the firms on the Fortune 500 list of biggest American companies by gross revenue operate tax haven subsidiaries in countries like Bermuda, Ireland, Luxembourg and the Netherlands. The Citizens for Tax Justice and the U.S.