— -- Investigators say they are "days, not hours" away from knowing the cause of a mysterious inferno that devastated an Annapolis, Maryland, mansion -- and the fate of four children and their grandparents feared to have been inside the home also remains unknown.
"The entire 16,000 square feet has collapsed, so we'll be doing an extensive digging operation to get down and try to locate not only the origin of the fire, the cause of the fire, [but also] any of the unaccounted for victims," said Capt. Robert Howarth of the Anne Arundel County Fire Department Investigations Unit.
"The focus is to make the building safe enough for personnel to enter to start conducting a search," Anne Arundel County Fire Capt. Russ Davies told ABC News.
However, crews may not be able to get into the house until Wednesday because there are unstable walls, water in the basement, and "structural beams that may need to be removed," Davies said. In addition, there continue to be small active fires burning at the scene, officials told reporters this morning.
Records list the home's owners as Don Pyle and Sandra Pyle, The Associated Press reported, though Davies said officials will not release information about the home's occupants until they are located.
A relative told ABC News she believes the Pyles and the couple's four grandchildren were in the house before the fire.
Don Pyle is the chief operating officer for ScienceLogic, the company's chief marketing officer, Yama Habibzai, told ABC News today.
"As you have heard, there was a fire at his home Monday morning," Habibzai said in a statement. "Unfortunately, we have no further information at this time. In respect to his family and their privacy, we are not providing any interviews."
"The last known location for the six [missing family members] was inside the house," Davies said. "So the house is the primary investigation."
Davies said officials interviewed neighbors who "gave us conflicting accounts" of the whereabouts of the family.
"The best information we have from the family is they were in the house overnight," Davies added.
Davies said this is the longest he's gone without answers on a case "in recent memory."
"The standard house [is] 2,500 square feet," Howarth said. "So you're looking at five standard houses put together. Six people in five standard houses, not a lot. Put them in one big house like this, this is more like a commercial fire than it is a residential fire."
Crews responded to a fire at the home at about 3:30 a.m. Monday, officials said. The fire, which reached four alarms, was reported by the alarm company and a 911 call from a neighbor. Officials said 85 firefighters responded to the scene and the blaze was brought under control in about 3.5 hours.
Records show the house was built in 2005 with a value listed at $4.2 million, according to the AP. The home did not have a sprinkler system because it was built prior to local sprinkler requirements, officials said.
Neighbors nicknamed the house "the castle," according to The Baltimore Sun, and the family's mailbox is even in the shape of a castle.
The Pyles hosted parties often, a neighbor told The Sun.
The Baltimore Field Division of the Bureau of Alcohol, Tobacco, Firearms and Explosives was also assisting at the scene. Agent Bill McMullan said ATF investigators from the national response team, who are "specially trained to do work on large fires," were expected to arrive at the scene today "to add to our Baltimore resources."
The national team hopes to enter the house Wednesday morning to begin a "very systematic process" that includes forensic mapping, photographing the scene, and releasing a canine "specially trained to detect accelerants."
The Maryland State Fire Marshall's Office was also assisting with the investigation, according to ATF.
ABC News' Matthew Larotonda contributed to this report.