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U.S. forces may be close to unlocking the mystery of who is behind a deadly innovation in Iraqi insurgents' weapons, a "lob bomb" now being used in Baghdad to target U.S. and Iraqi combat outposts, a senior American general said Friday in an Associated Press interview.

Maj. Gen. Jeffery Hammond, commander of U.S. forces in Baghdad, called the weapon "the greatest threat right now that we face," and he likened the shadowy group behind it to the American military's elite Delta Force.

The weapon is particularly worrying because it is designed to cause catastrophic damage and cannot be stopped once it has been launched, Hammond, commander of …