(CNN) -- Some people who didn't file a federal tax return for 2006 have more than 1 billion reasons to reconsider, but they need to do it fast, according to the Internal Revenue Service.
The IRS says it has more than $1.3 billion in refunds for 1.4 million people who didn't file a 2006 return. But to collect the cash, they will have to file for that year by April 15.
Otherwise, the money will become the property of the U.S. Treasury, as happens with all tax refunds that aren't claimed within three years.
The IRS estimates the median unclaimed refund for 2006 nonfilers is $604. California is the state with the most nonfilers who qualify for refunds from that year (159,700), with Texas (109,600) and Florida (101,700) right behind.
Some of the eligible people may have skipped filing because they earned too little to require it, even though they had taxes withheld from their wages or made quarterly estimated payments, according to the IRS.
"A lot of those are dependents or younger people ... with part-time jobs" -- people who didn't make enough to require filing, but who may have underestimated the refunds they'd receive if they did file or didn't realize they were eligible for certain credits, said Elaine Smith, enrolled agent and master tax adviser at H&R Block.
Others include people who were required to file, but didn't. People who failed to submit a required return but are owed refunds will not be penalized for filing that return late, according to the IRS and Smith.
"There's no real penalty for letting them keep your money for an extra amount of time," Smith said, "but if you owe [for that year instead of qualifying for a refund], then they will certainly penalize you and charge you interest."
The $1.3 billion doesn't include credits for which the nonfilers may qualify, including the earned income tax credit, which goes to people whose incomes are under certain levels.
People who don't file a 2006 return also would miss the one-time telephone excise tax refund, intended for most telephone customers, including most cell phone users. The refund, available only on the 2006 return, applies to long-distance excise taxes billed from March 2003 through July 2006, according to the IRS.
A standard telephone excise tax refund runs from $30 to $60, though the credit can be higher.
Other big credits that nonfilers may be neglecting are those for college expenses, Smith said.
"They might think, 'I have only a [small amount] withheld,' but they don't know they're eligible for all these other things," she said.
Current- and prior-year tax forms are available on the Forms and Publications page of IRS.gov. People seeking the forms also can request them at 1-800-829-3676.
CNN's Jason Hanna contributed to this report.