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The Obama administration said it was encouraged by data showing that only about 10 percent of those who completed the modification process by the end of March had missed at least two months of mortgage payments.

“The majority of homeowners are maintaining,” their payments, said Tim Massad, acting assistant Treasury secretary for financial stability. That, he said, reflects “rigorous standards the program uses to provide assistance to responsible homeowners.”

Low participation means that the program is likely to cost far less than originally forecast. Though Treasury has set aside $50 billion from the federal bank bailout fund for the housing relief effort, only about $483 million has been spent, Mr. Barofsky said.