HOUSTON—Investigators have recovered a weapon that may have been used to kill a 5-year-old Houston boy whose body was allegedly kept in a storage unit before being discovered in an East Texas motel. The weapon was being tested to see if it was the one used to kill Samuel Olson, Andrea Beall, a prosecutor with the Harris County District Attorney’s Office, told reporters Thursday. “We’re not going to stop until we get justice for little Samuel,” said Beall, who declined to say what type of weapon was found or used to kill the boy. The disclosure came the same day a third person was charged in the case. All three face evidence tampering counts and none have been charged with murder, though prosecutors said earlier this month that they expected to bring additional charges against one of them, Theresa Balboa, who was Samuel’s father’s girlfriend. Samuel’s cause of death was …