Democratic presidential candidates have seized on reports that Amazon.com Inc. paid no 2018 federal taxes as they press for changes in the tax system.

A closer look at the internet giant’s tax disclosures over several years paints a more complicated picture: Amazon has paid income taxes somewhere, albeit at a low rate, likely helped by deductions and incentives related to investment, research and employee compensation.

Earlier this week, former Vice President Joe Biden spotlighted the company’s tax payments during a campaign stop in Iowa, this week, saying it should at least pay some taxes. He tweeted Thursday: “No company pulling in billions of dollars of profits should pay a lower tax rate than firefighters and teachers.”

Amazon tweeted back: “We pay every penny we owe. Congress designed tax laws to encourage companies to reinvest in the American economy. We have. ... Assume VP Biden’s complaint is w/ the tax code, not Amazon.”

Sen. Elizabeth Warren (D., Mass.) cited Amazon as she proposed a new corporate tax that would have cost the company nearly $700 million in 2018.