After spending a day at the beach, Mazen Fuqaha, a commander of Hamas’s military wing, dropped his wife and children at the entrance to their Gaza apartment block and went to park his car.

His life ended in a garage, where he was found dead in the driver’s seat of his car after sunset on Friday, shot four times in the head from close range. Hamas, the Islamic militant group that controls the Palestinian coastal territory of Gaza, blamed Israel. The group presented no evidence but said the “quiet assassination” bore the hallmarks of a hit by Mossad, Israel’s spy agency, and pledged to respond.

Israeli officials have not commented on Mr. Fuqaha’s death other than to say that he was involved in planning attacks against Israel. […]