WASHINGTON — A congressional committee has opened an investigation of the State Department’s inspector general, alleging he blocked fraud investigations in Afghanistan and Iraq, including potential security lapses at the newly built American Embassy in Baghdad.

Also under scrutiny is whether Blackwater USA, the private security firm banned this week from working in Iraq for the alleged killing of eight civilians, was “illegally smuggling weapons into Iraq,” according to a letter to IG Howard Krongard obtained yesterday by the Associated Press.

The Democrat-led investigation accused Mr. Krongard of trying to protect the White House and the State Department, telling him in the letter, “your partisan political ties have led you to halt investigations, censor reports, and refuse to cooperate with law enforcement agencies.”

Based on charges from a number of current and former senior investigators who worked for Mr. Krongard, the letter from the House Oversight and Government Reform Committee also questioned whether he adequately investigated illegal labor trafficking allegations involving the Kuwaiti company that was building the Baghdad embassy.

Ralph McNamara, the former deputy assistant inspector general for investigations, said in an Associated Press interview yesterday that he came forward with the allegations against his former boss because he was concerned that State Department employees would be at risk when working in the new embassy.

“A rocket — an unexploded munition — went through a portion of a cement ceiling there, and it was supposed to be an area able to withstand a direct hit from a missile that did explode,” Mr. McNamara said. He said investigators wanted to look into charges that the walls were not built to the required thickness or concrete consistency, but they were blocked from pursuing it.