There was nothing left after the inferno. Just ash and char and an awful mystery.

A $6 million Annapolis home so big that neighbors called it “the castle” was destroyed by flames in the wee hours of Monday morning. Its wealthy owners, computer networking executive Donald Pyle and his wife, Sandra, are missing and probably dead. So are their four grandchildren.

On Tuesday, as firefighters were still directing their hoses on the smoldering wreckage, investigators began trying to unravel the perplexing questions everyone has been asking:

How did this fire start? Why did a 16,000-square-foot home with every amenity not have an alarm or water sprinkler system capable of preventing the intense flames from consuming every square inch? How long will it take to find the six bodies that are probably lying in the ruins?

The answers could remain elusive for days or even weeks. Authorities said at a news conference that they are engaged in an “active criminal investigation,” although they cautioned that they had no reason to believe that it was a suspicious fire.

They declined to identify the possible victims, even though parents of the four children’s classmates at the Severn School were informed by the school’s headmaster in a letter that the children and their grandparents had died.

“We do not declare anyone deceased until we know for sure and have recovered evidence that a person is deceased,” said Capt. Robert Howarth, commander of Anne Arundel County fire department’s fire and explosives investigation unit. “We still do not have 100 percent proof that they are in this house.”

But the immolation was so complete that that proof may be hard to come by. A crane was brought to the six-acre property along Church Creek on Tuesday to help investigators sift through the wreckage. A basement the size of a ballroom was filled with soot, smoke and water.

Anne Arundel fire spokesman Russ Davies said it could take days to remove debris and search for the missing family, who were already being mourned by relatives, neighbors and colleagues.

Cars filled the driveway at the address listed for Randy and Stacey Boone, the parents of two of the children, about a 15-minute drive from the Pyle mansion. As people streamed in and out of the two-story house, a man standing outside said they appreciated the news media’s concern but declined to comment.

Howarth said the investigation, which will include a national response team from the Bureau of Alcohol, Tobacco, Firearms and Explosives, is complicated by the mansion’s size and construction. Seven-ton steel beams will have to be moved. Crews had to fight the blaze as if it were “more of a commercial fire than a residential fire,” he said, because the mansion is about the size of five standard single-family homes combined.

Jim Milke, professor and chair of the department of fire protection engineering at the University of Maryland, said smoke detectors — if operating properly — should have given the family ample time to escape if they were in the home at the time. Maryland has required smoke alarms in residential dwellings for about 30 years, he said.

“How does a fire get to this extent if it has the proper protection in it?” Milke said.

Had sprinklers been in the home, the blaze could have been contained to a single-alarm response instead of the four-alarm fire that consumed the mansion, Milke said. He also said getting to the root of the fire — and determining whether any foul play was involved — will be particularly difficult for investigators based on the photos he has seen of the aftermath.

“With the amount of the collapse, it could be that those [victims] are well under the rubble,” if they were in the home, Milke said.

Caroline Wugofski watched from a safe distance Tuesday as smoke still curled up from the home’s foundation.

Wugofski, 77, has lived next door to the Pyles since the couple moved in a decade ago. They were a generous, charitable family, she said, always hosting neighbors and friends at their mansion, which was designed to look like an English-style castle complete with a suit of armor inside, she said. The mansion was so eye-catching that strangers on boats would sometimes pull alongside the home believing it was a country club and asking where the golfing greens were.

The Pyles were family-oriented, with children often running around the property, Wugofski said. The couple bought each of their sons a home in a nearby neighborhood, she added.

They were loyal Orioles and Ravens fans, with many of their fundraisers focused on supporting the teams, Wugofski said. The family had an orange Orioles pool table, and Sandra Pyle once dyed her hair purple to support the Ravens.

“It’s a shock,” Wugofski said. “You just don’t expect something like this to happen to people you know.”

At the Severn School in Severna Park, where one of the parking spaces is marked “Reserved for Pyle Family,” classes were canceled Tuesday for the lower school. In his letter to parents, Headmaster Doug Lagarde urged the school community to pull together.

“In all kinds of ways it is too soon to make any sense of this tragedy, and we may never,” he wrote, “but it is in times of deep sadness that a community shows its true self.”

Donald Pyle, 56, who grew up north of Baltimore and graduated from the University of Delaware, had just become chief operating officer of ScienceLogic, an IT company in Northern Virginia. But, as he told The Washington Post in an interview in October, he made his fortune in “sales, very high-ticket sales to telephone companies and other enterprises.”

In the booming-tech 1990s, he got in on the ground floor of Silicon Valley venture-backed companies, including Stratacom and Juniper Networks. He later became chief executive of Lauren Networks in Pittsburgh and Netcordia in Annapolis. In October, he joined ScienceLogic, where colleagues declined to comment.

Dave Donovan first met Pyle when they played on the lacrosse team at Dulaney High School in Baltimore County. A close friend of the family, Donovan got his first job from Pyle’s father, working in the parts department of the family’s forklift business.

The two played lacrosse at Delaware but fell out of touch after graduation. They met by chance in an airport about seven years ago, and Donovan became a consultant for the company where Pyle worked at the time.

Donovan, the founder and president of Aequus, an IT company in Great Falls, Va., described Pyle as a man who was quick to laugh and enjoyed spending his free time on the water or the golf course. Pyle also hosted fundraisers at the mansion on Childs Point Road to benefit the Delaware lacrosse team, Donovan said.

Though he found financial success, Pyle’s demeanor never changed, Donovan said.

“I never, in my whole time with Don, had an argument,” he said. “He was the same happy-go-lucky, optimistic guy before he made the money as he was when he made the money.”

On Tuesday afternoon, Will Frece stopped outside the mansion’s gates to drop off an orange bouquet at a growing memorial.

Frece, an IT architect at Morgan Stanley, said he traveled from New York to visit the scene of the fire. He had worked as an intern at Netcordia from 2007 to 2009 and considered Pyle his mentor.

“He was a very, very driven individual,” Frece said. “His expectations were very high.”

Frece called Pyle “a visionary in his space.” He had a rare understanding, Frece said, of both business and technology.

“Don sat at the intersection of the two better than anyone I’ve ever known,” he said.

Frece said he had been to several parties at the home and had once met Sandra Pyle, 63, describing her as “very hospitable.”

“They were festive people,” Frece said. “Very, very upbeat.”

Seeing the once-magnificent home in ruins, Frece said, was surreal. “It’s just bizarre,” he said. “It was a palace.”

John Woodrow Cox, Steven Overly, Dana Hedgpeth, Amrita Jayakumar and Julie Tate contributed to this report.