Next tonight, the mansion fire mystery. The multimillion dollar home burning to the ground in Maryland. The family, grandparents and grandchildren missing for a time. ABC's bazi kanani on what authorities now believe. Reporter: Tonight, federal investigators are trying to determine what sparked an inferno at this Maryland mansion. We got visible fire shooting up from the ground. Reporter: Firefighters still extinction wishing hot spots after the four-alarm fire brought this 16,000 square foot mansion to the ground. There are six people unaccounted for. Reporter: Tech executive don Pyle, his wife sandy, and four young grandchildren visiting Sunday night, believed to have been inside during the blaze, all missing. The construction of the house contained a lot of very heavy materials. We're looking at some steel beams that weigh in the area of seven tons. Reporter: From here, you can still smell the smoke. The fire still smoldering on this enormous property, nearly 40 hours after it began. Reporter: The fire here reported at 3:30 in the morning. Experts say more than half of deadly home fires occur overnight, spreading fast, leaving only minutes to escape. And they were the most generous and loving people in the world. Reporter: At the Gates of this moment, a makeshift memorial as this family waits for word of their loved ones. Bazi kanani, ABC news, annapolis, Maryland.