An Interior Department official says he was forced to resign because he challenged department assertions that it is repairing a historically mismanaged trust fund for American Indians.

Special Trustee Thomas Slonaker, whose position was created by Congress to provide independent oversight for overhaul of the fund, submitted his resignation to Interior Secretary Gale Norton on Tuesday.

"I was given the choice of resigning or being fired," Slonaker said in an interview. "Things have not been going well in terms of trust reform, but it's not always the message they want to hear."

Slonaker has clashed with Norton and department officials, offering testimony in court and before Congress that contradicted assertions of progress toward fixing the century-old trust fund designed to manage oil, gas, mining and timber royalties from Indian land.

A history of mismanagement going back many decades has resulted in the loss of an unknown amount of money. Attorneys for Indians who sued the government say at least $10 billion is owed to more than 300,000 Indian landowners.

Last week, White House counsel and Justice Department attorneys urged Slonaker not to submit prepared testimony to a Senate Indian Affairs Committee hearing in which he challenged the department's plans to account for lost Indian money.

The department has told Congress it will take $2.5 billion and 10 years to conduct a full accounting, but Slonaker said a complete accounting is impossible because records are missing or have been destroyed.

He said he was given a letter of resignation to sign during a meeting with Norton and Deputy Secretary Steven Griles Tuesday afternoon.

Norton appointed Donna Erwin, the No. 3 official in Slonaker's office, as a temporary replacement and in a statement thanked Slonaker for his service.

Interior Department spokesman Eric Ruff would not comment on Slonaker's claims.

During a hearing Tuesday before the Senate Indian Affairs Committee, American Indian leaders said they won't back down on demands that an independent commission supervise the Interior Department's management of $1 billion a year in royalties from Indian land.

Tribal leaders want the commission to have the power to subpoena documents, audit the department's accounting of the royalties and impose fines against the interior secretary to repair a history of mismanagement that has squandered an unknown amount of money.

Griles said the department has constitutional concerns about creating an independent commission with oversight of a cabinet secretary.

The impasse probably means legislation meant to fix the historically mismanaged trust fund won't pass Congress by the end of the year.

The government has managed proceeds from Indian tribal land since 1820 and for individual Indians since 1887. Today it controls 45 million acres of land belonging to 315 tribes and 11 million acres for more than 300,000 individual Indians. The lands generate more than $1 billion annually.