WASHINGTON — There was no hint of it in the bland resignation letter, but it was Blackwater USA that did in the State Department’s security chief.

The storm over last month’s deadly Blackwater shooting in Baghdad and the fury over the department’s security practices in Iraq claimed their first Washington casualty yesterday, forcing the assistant secretary of state for the Bureau of Diplomatic Security to step down.

Richard Griffin’s resignation came amid growing questions about the use of private contractors and was not voluntary, according to officials familiar with the circumstances of his departure.

He had been under withering criticism for the agency’s lax oversight of private guards who protect American diplomats in Iraq, notably those from Blackwater accused of killing 17 Iraqi civilians on September 16.

Mr. Griffin made no mention of the furor in his letter to President Bush and Secretary of State Rice, but his departure comes as the department is struggling to cope with the consequences.

It followed by just a day the release of a report commissioned by Ms. Rice that found serious lapses in the department’s oversight of private guards, who are employed by and report to the bureau where Mr. Griffin had worked.

Ms. Rice accepted the resignation, which is effective November 1. A State Department spokesman, Sean McCormack, said Mr. Griffin will be replaced on an acting basis by one of his deputies, Gregory Starr.

“I just want to thank him for his exemplary service to the country,” Mr. Rice told reporters before meeting with Deputy Prime Minister Barham Salih of Iraq.

Mr. Griffin, an ambassador-rank official who was previously deputy director of the U.S. Secret Service and inspector general for the Department of Veterans Affairs, had been in his current job since June 2005. He served 36 years in the American government, according to his official biography. In other news, the wars in Iraq and Afghanistan could cost as much as $2.4 trillion through the next decade, the nonpartisan Congressional Budget Office said yesterday. The White House brushed off the analysis as “speculation.”

The estimate was the most comprehensive and far-reaching one to date. It factored in costs previously not counted and assumed that large number of forces would remain in the regions.

According to analysis, America has spent about $604 billion on the wars, including $39 billion in diplomatic operations and foreign aid.

If America were to reduce the number of troops deployed to Iraq and Afghanistan to 75,000 six years from now, it would cost the government $1 trillion more for military and diplomatic operations and $705 billion in interest payments to pay for the wars through 2017.

Democrats, who say voters will not stand for it, would consider paying for the military campaigns in short installments, instead of full one-year terms.