Anas Abu Shawish, who was a militant with al-Qassam Brigades, died following a tunnel collapse in Tuffah, a district of Gaza City, shortly after midnight, according to a statement from the group. The statement didn't say what caused the collapse.
Three people also were injured in the collapse, it said.
Hamas uses tunnels underneath Gaza as a strategic weapon - to store arms, hide during wars and carry out attacks using tunnels to Israel.
Over the past three years, more than 20 militants have died under similar circumstances, according to Gaza rights groups.
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