Unclaimed federal income tax refunds totaling more than $8.5 million may be waiting for an estimated 7,000 Hawaii taxpayers who did not file a 2015 federal income tax return, according to the Internal Revenue Service.

“To collect the money, these Hawaii taxpayers must file their 2015 tax return with the IRS no later than this year’s tax deadline, Monday, April 15,” IRS spokesman David Tucker II said.

Overall, unclaimed income tax refunds totaling almost $1.4 billion may be available for an estimated 1.2 million taxpayers who did not file the 2015 return, the IRS said.

“Students, part-time workers and many others may have overlooked filing for 2015. And there’s no penalty for filing a late return if you’re due a refund,” IRS Commissioner Charles Rettig said.

The IRS estimates the midpoint for the potential refunds for 2015 to be $879 — meaning half of the refunds are more than $879 and half are less.

In cases where a federal income tax return was not filed, the law provides most taxpayers with a three-year window of opportunity to claim a tax refund. If they do not file a tax return within three years, the money becomes the property of the U.S. Treasury. For 2015 tax returns, the window closes April 15, 2019, for most taxpayers. The law requires taxpayers to properly address, mail and ensure the tax return is postmarked by that date.

The IRS reminds taxpayers seeking a 2015 tax refund that their checks may be held if they have not filed tax returns for 2016 and 2017. In addition, the refund will be applied to any amounts still owed to the IRS or a state tax agency and may be used to offset unpaid child support or past due federal debts, such as student loans.

By failing to file a tax return, people stand to lose more than just their refund of taxes withheld or paid during 2015. Many low- and moderate-income workers may be eligible for the Earned Income Tax Credit (EITC). For 2015, the credit was worth as much as $6,242.

Tax forms and instructions are available on the IRS.gov Forms and Publications page or by calling toll-free 800-TAX-FORM (800-829-3676).