The White House remained silent on President Barack Obama's choice of a successor for the commissioner of the Woodlawn-based Social Security Administration, who announced Monday he would step down next month.

Michael J. Astrue, an appointee of former President George W. Bush, said he intends to return to Massachusetts after completing his six-year term, which expired earlier this month. It's unclear who will lead the agency when he leaves.

"I consider it a great privilege to have led this remarkable agency for six years," Astrue, 56, said in a statement.

Neither the Social Security Administration nor the White House would provide information on Astrue's successor. The governing statute indicates that Astrue's deputy, Carolyn Colvin, could serve in his place, at least until Obama nominates a new commissioner. That person must be confirmed by the Senate.

Colvin is a former secretary of the state Department of Human Resources and served as special assistant to Maryland's secretary of transportation. Her term also expired this month.

Astrue served longer than any Republican commissioner and has the third-longest tenure in the agency's history, according to the Social Security Administration.

Under Astrue's leadership, the agency said, 6 percent of disability claims were subject to new fast-track procedures and the time to receive a disability hearing dropped by 180 days to 360 days, despite an increased number of claims and fewer resources. A new state-of-the-art facility is set to open next year to replace an outdated data center, among other improvements.

More than 11,000 people work at the agency's headquarters in Baltimore County.

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