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 taxes if they repatriated the funds, according to a study released Tuesday.

The study, by two left-leaning nonprofit 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 Center for Tax Justice and the U.S. Public Interest Research Group Education Fund used the companies' own financial filings with the Securities and Exchange Commission to reach their conclusions.