with that mysterious mansion fire. Federal investigators are on the scene in Maryland trying to determine what caused the massive blaze and they are not ruling out foul play. ABC's bazi kanani has that story. Reporter: This morning, federal investigators getting their first up-close look at the charred evidence as they try desperately to figure out what caused the intern another at this 16,000-square-foot Maryland mansion, a crime scene at least five times the size of an average house. The amount of evidence that we're going to be collecting and analyzing, there's going to be a lot of that because of how big it is. Reporter: Don Pyle, his wife sandy and four visiting grandchildren still unaccounted for. Believed to be inside the home when fire alarms sounded at 3:30 A.M. Monk. It was the home security company that called for help. The four-alarm blaze raging uncontrollably in part because the $9 million mansion didn't have fire sprinklers. We have no witness that can tell us where this fire started. Reporter: ABC news learning there were no reports of previous trouble at the home and no records of recent lawsuits or serious financial trouble. But the ATF speaking exclusively with ABC news says it's still too early to rule out foul play. We're going to sift through pieces of evidence. Every piece in there room by room. So that's going to take time. I don't believe it's happened. I mean even staring at it it's not real to me yet. Reporter: Grieving friends are placing flowers at the estate say they can't imagine anyone wanting to hurt Pyle or his family. My experience working with him was a couple of the best years of my life. He was a terrific leader. Reporter: This morning, some of the debris is still smoldering but do expect there will be some areas safe enough for investigators to enter. At this point they say it's too early to have an opinion about whether this was a cruel arson or a horrific dent.