The State Department is being accused of blocking a key congressional probe into the private military firm Blackwater USA. On Tuesday, House Oversight Chair Henry Waxman said the State Department had told Blackwater not to turn over documents without prior approval. Blackwater is under scrutiny after its forces killed at least 11 civilians last week in Baghdad. It’s also under investigation for alleged weapons smuggling into Iraq. The State Department called Waxman’s complaint a “misunderstanding.”