A huge blast late Tuesday destroyed a training base run by Hamas in southern Gaza, and at least five people were hurt, witnesses and a health official said. It was not immediately known what caused the blast. Hamas had no comment. Dr. Moaiya Hassanain of the Palestinian Health Ministry said two of the five wounded Hamas militants were in critical condition. Witnesses said the blast destroyed the base, and a fire was burning there an hour afterward. They said a second, smaller blast was heard about 15 minutes after the first one.