Federal income tax refunds totaling more than $21 million could be lost by about 22,000 Louisiana taxpayers who did not file a federal income tax return for 2011.

The law provides most taxpayers with a three-year window of opportunity for claiming a refund. To collect the money, these taxpayers must file a 2011 tax return with the Internal Revenue Service postmarked no later than April 15, the agency said.

The median refund is $693 — meaning half are less than that amount and half are higher.

“People could be missing out on a substantial refund, especially students or part-time workers. Some people may not have filed because they didn’t make much money, but they may still be entitled to a refund,” said IRS Commissioner John Koskinen.

There is no penalty for filing a late return that qualifies for a refund. If no return is filed, the money becomes property of the U.S. Treasury.

The IRS also reminds taxpayers seeking a 2011 refund that their checks may be held if they have not filed tax returns for 2012 and 2013. In addition, the refund will be applied to any amounts still owed to the IRS, or their 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 also stand to lose more than just their refund of taxes withheld or paid during 2011. Many low- and moderate-income workers may not have claimed the Earned Income Tax Credit. For 2011, the credit is worth as much as $5,751, the IRS said.