The IRS is going after high-income earners who skipped out on filing federal income tax returns in more than 125,000 instances since 2017, the agency said Thursday.

They’re believed to owe, based on a conservative estimate, hundreds of millions of dollars, the IRS said. The number could be much higher – but the IRS said it can’t be sure of an exact amount since the agency doesn’t know what potential credits and deductions these people may have.

“At this time of year when millions of hard-working people are doing the right thing paying their taxes, we cannot tolerate those with higher incomes failing to do a basic civic duty of filing a tax return,” IRS Commissioner Danny Werfel said in a press release. “The IRS is taking this step to address this most basic form of non-compliance, which includes many who are engaged in tax evasion.”