WASHINGTON, May 10 — An investigation by the Justice Department ethics office into the conduct of department lawyers who approved the National Security Agency's domestic surveillance program has been closed because investigators were denied security clearances, according to a letter sent to Congress on Wednesday.

The head of the department's Office of Professional Responsibility, H. Marshall Jarrett, wrote in the letter to Representative Maurice D. Hinchey, Democrat of New York, that "we have been unable to make meaningful progress in our investigation because O.P.R. has been denied security clearances for access to information about the N.S.A. program."

Mr. Jarrett said his office had requested clearances since January, when it began an investigation, and was told on Tuesday that they had been denied. "Without these clearances, we cannot investigate this matter and therefore have closed our investigation," the letter said.

Mr. Hinchey said the denial of clearances was "hard to believe" and compounded what he called a violation of the law by the program itself, which eavesdrops without court warrants on people in the United States suspected of ties to Al Qaeda.