Federal employees, retirees and beneficiaries are missing out on more than $88 million in abandoned and forgotten Thrift Savings Plan accounts.

Thousands of accounts with amounts ranging from hundreds to tens of thousands of dollars sit unclaimed within the TSP – and it's been rising every year, according to Federal Retirement Thrift Investment Board.

Close to $6 million was "abandoned" in 2010 – but that amount grew to $26 million in 2014, according to agency numbers. Since 2010 more than $68 million has been abandoned and unclaimed – and a total of $88 million has been abandoned overall.

If a TSP account holder does not make a withdrawal choice for their funds by the time they are 70.5 years old, the agency sends them repeated letters and warnings before "abandoning" the account.

Abandoned accounts do not include what are called "inactive" accounts, which are made up of participants who have terminated contributions or had them suspended because they are making hardship withdrawals, according to the agency. Inactive accounts are never abandoned, according to the agency.

If a federal employee or retiree dies without naming a beneficiary or notifying family about a past TSP account, those funds could sit forgotten for years.

But a participant can reclaim the account by contacting the TSP and can then access the money. Beneficiaries or descendants can also claim abandoned funds by providing the proper documentation, according to the agency.

And hundreds of account holders or beneficiaries reclaim millions of dollars a year. Since 2010, more than $262 million in TSP accounts were "abandoned" – but $194 million was subsequently restored, according to the agency.

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The total amount of abandoned money is a small fraction of the $450.6 billion in TSP funds spread across 4.7 million accounts, but it could mean a lot of money for a retiree or beneficiary.