LibriumArcana.Games

WASHINGTON (CNN) -- The State Department's chief of diplomatic security has resigned amid scrutiny of the use of private military contractors to guard the department's staff in Iraq, a spokesman said Wednesday.

Richard Griffin, a 36-year career official, gave no explanation for his decision in his resignation letter to President Bush.

"I am grateful that I had the opportunity to serve as the assistant secretary for the Bureau of Diplomatic Security, whose brave men and women serve on the front lines of the Global War on Terror," he wrote in his resignation letter, a copy of which was obtained by CNN.

"Without their courage and commitment, the State Department could not possibly carry out its foreign policy mission."

Secretary of State Condoleezza Rice accepted Griffin's resignation, State Department spokesman Sean McCormack said.

Griffin's decision to step down follows weeks of investigations into the September 16 killings of Iraqi civilians by Blackwater USA, a North Carolina-based firm hired by the State Department to protect American diplomats in Baghdad.

Blackwater founder and CEO Erik Prince says the guards came under fire while protecting a State Department convoy and acted properly in self-defense.

But Iraqi authorities have called the killings "premeditated murder" and demanded Blackwater leave Iraq.

A State Department report of its own operations, released Tuesday, found lax oversight of security contractors, who are under the supervision of the Bureau of Diplomatic Security.

It recommended tightening the rules of engagement for contractors to bring them in line with those used by the U.S. military, and Rice appointed a top U.S. diplomat to oversee all security operations in Iraq.