BankUnited FSB appeared on the verge of being shut down this week, after its federal regulator rejected the bank's plan to raise capital and ordered it to sell itself or most of its assets within 20 days. The Office of Thrift Supervision's prompt corrective action, which comes after months of trouble for the subsidiary of BankUnited Financial Corp. ( BKUNA), requires management to make strong efforts to sell itself and to cooperate in OTS efforts to "market" the institution. The OTS also said that its directive was issued in order to "resolve the institution's problems at the least long term cost to the deposit insurance fund," referring to the costs that would be incurred by the Federal Deposit Insurance Corp. if BankUnited were to fail. The Coral Gables, Fla.-based thrift was included in TheStreet.com's list of undercapitalized banks and thrifts published last week. It was considered critically undercapitalized by regulatory standards, with a Tier 1 leverage ratio of just 1.37% and a total risk-based capital ratio of 3.60%. These ratios need to be at least 5% and 10% for a bank or S&L to be considered well capitalized under ordinary regulatory guidelines. The institution's nonperforming assets, including loans past due 90 days or in nonaccrual status and repossessed real estate comprised 9.95% of total assets as of Dec. 31, and it's net charge-offs (actual loan losses) for 2008 totaled $438 million, or 6.12% of average loans. Net losses for the year totaled $1.1 billion. BankUnited's problems sprang from its concentration in option-payment adjustable-rate mortgages, one of the worst mortgage products to proliferate during the housing boom. These loans -- known as option-ARMs -- gave the customer several monthly payment options early in the life of the loan. While options varied, they generally included a high option to make an ordinary amortized payment of principal and interest, a middle "interest only" option to pay just the last month's accrued interest, and a low option to pay an amount less than the last month's accrued interest. It was this last option that caused all the problems, since if a customer pays less than the accrued interest, the unpaid interest gets tacked on to the loan principal balance. This is known as negative amortization.