In the recent failure of Pasadena, Calif.-based IndyMac Bancorp, confusion over trusts was one reason some found bank accounts uninsured.

The FDIC immediately said it would make available 50 percent of some $1 billion in potentially uninsured deposits.

Nevertheless, the misunderstanding of how the FDIC covers trust deposits is a growing problem, in part because of the rapid growth of trusts as estate planning tools.

If you have an estate plan and invest in bank deposits, the FDIC encourages you to give the agency a call at 1-877-275-3342 to discuss your questions. Ask to speak to a deposit insurance trust specialist.

Even if all your deposits are federally insured against a bank failure, determine how that could change if you or your spouse dies.

In fact, it also could pay to check with the FDIC periodically on its coverage of bank deposits inside your trusts because the rules change.

With a revocable trust, which is a trust you can control, insurance coverage to $100,000 is provided for each beneficiary. Catch: The beneficiaries must be a spouse, child, grandchild, parent or sibling. Blended families, such as stepchildren and adopted children, also are allowed. Nieces and nephews are not.

Once a trust becomes irrevocable, which frequently happens when somebody dies, different FDIC rules kick in. No longer must a beneficiary be a specific type of relative.

But "irrevocable trusts often cannot be FDIC-insured for more than $100,000," explained Martin Becker, senior consumer affairs specialist in the FDIC's deposit insurance section.

So say a husband and wife with $400,000 set up a popular "AB Trust," to spare their two children from getting zapped with taxes.

When one spouse dies, subtrusts are created. One trust with $200,000 would be a survivor's trust and that remains revocable. So the surviving spouse can continue to make changes to the account and have the qualified beneficiaries covered by FDIC insurance. The "B" trust, on the other hand, with its $200,000, becomes irrevocable.

Under FDIC rules, if a bank fails within six months of the death of either spouse, the trust is treated as if both spouses are still alive. So $400,000 would be fully covered.

But after six months, if the "B" trust becomes irrevocable, one major FDIC issue becomes whether the children's interests under the B trust are "contingent." Often, they are.

Does your B trust have language that makes the irrevocable trust contingent upon letting the trustee reallocate funds to a beneficiary for medical needs? Or, does it allow the surviving spouse to invade the principal? If so, your irrevocable trust has contingencies that could disqualify beneficiaries from added FDIC insurance coverage.

This means only $300,000 -- the $200,000 in the revocable trust plus $100,000 for the irrevocable trust -- may be covered by FDIC insurance.

If you have an AB trust or another type of trust that could be irrevocable, you easily can avert this issue. Simply split your deposits up among different FDIC-insured banks, Mr. Becker notes.

Unlike with irrevocable trusts, the FDIC does not look for contingencies with revocable trusts.

Have CDs in your trust agreement with a broker rather than a bank? Be sure your broker has a copy of your trust agreement, Mr. Becker advises.

"The broker would be responsible with providing the FDIC with documentation," he said.