TAMPA A Tampa executive "systematically" executed his wife and two teenage children in their bedrooms before splashing gasoline throughout the family's mansion, shooting himself in the head and burning down the house, authorities said Friday.The official — albeit still incomplete — account of the inferno that consumed the $1 million home in the exclusive Avila community north of Tampa this week confirmed what was widely suspected: Darrin Campbell, 49, was responsible for the deaths of his wife, Kimberly Campbell, 51, and children Colin Campbell, 18, and Megan Campbell, 15."This has been determined to be a murder-suicide," Hillsborough County sheriff's Col. Donna Lusczynski said at a media briefing Friday afternoon. Darrin Campbell, she said, "systematically shot his son, his daughter and his wife in the head. He then placed fireworks throughout the residence, used an accelerant to assist in lighting the fire, lit the fire, and then shot himself."But if the facts of the crime have been established, the motive remains a mystery. "At this point," Lusczynski said, investigators "don't know why this occurred."According to those who knew them, the Campbells projected a seemingly flawless image of wealth and probity. Darrin Campbell, the chief operating officer at a local digital records company, previously was an executive at major corporations including Pabst Brewing in Texas and Anchor Glass Container in Tampa.The Campbell children attended the private Carrollwood Day School, where Colin was a well-known member of the varsity baseball team. (At a playoff game Friday, players for the girls' varsity softball team wore jerseys with Colin's and Megan's initials on the sleeves.) Darrin Campbell had served for nine years as the school's volunteer treasurer. They were leasing the five-bedroom home in Avila, known for its luxurious mansions and heavy security, from retired tennis pro James Blake."We're just all in such shock here and trying to understand what would make Darrin go to the dark side like that," said Martha Stein, a neighbor of the Campbell family at their previous home in Westchase, where they lived for more than a decade."The Campbells were the perfectly normal and perfect suburban, affluent family," Christopher Barrett, a Westchase resident who publishes the World of Westchase community newspaper, wrote in an email.The family won the newspaper's holiday decorating contest for four years straight beginning in 2008, Barrett said. They collected donations from visitors, making a $3,733 contribution in 2009 to Metropolitan Ministries.But some aberrant behavior had begun to surface that could point to problems in Darrin Campbell's career and finances.In February 2006, Darrin and Kimberly Campbell bought a vacant lot in Odessa for $294,000. In March 2011, the local property owners' association placed a lien on the lot for unpaid association fees totaling $8,000, which were apparently paid by February 2012.In both 2008 and 2012, Darrin Campbell was delinquent in paying his property taxes on the lot, though the taxes, totaling about $7,800, were eventually paid. Taxes on the property for 2013 have not been paid."We are conducting a financial audit on him to determine if there were some financial issues," Lusczynski said. "We do know that he was working up to a point but recently had taken some time off."Campbell took last week off from his job at VASTEC, the digital records company. On Monday, a VASTEC spokesman said, he requested a leave of absence "for unspecified reasons," which the company approved.The gun Campbell used in the murders and suicide was a .40-caliber Sig Sauer, a popular handgun he bought in July 2013 at Shooters World, a store and gun range on Fletcher Avenue. Bruce Kitzis, the store's general manager, said Campbell's only visit there was to buy the gun, and that he passed a federally mandated background check before purchasing the weapon."It was (about) a year ago, was the only time he came in. We did the background check and everything was fine," Kitzis said. "It's unfortunate. We at Shooters World did everything we could, of course. There are so many wonderful gun owners. It's a tragedy."Authorities' description of the mysterious murder-suicide and arson came as another bewildering case that shattered an affluent Tampa neighborhood is also in the news. Julie Schenecker, the New Tampa mother who fatally shot her teenage son and daughter in 2011, is now on trial in Hillsborough Circuit Court on two charges of first-degree murder.It is unclear exactly what led investigators, more than two days after the Campbells' home went up in flames, to conclude that Darrin Campbell had killed his family and then himself.Lusczynski said deputies have not yet discovered a suicide note, although they are analyzing documents and computers found in the burnt remains of the house at 16223 Sierra de Avila."This is certainly disturbing," Lusczynski said. "You have two teenagers that were well-respected, well-liked by their friends, their peers and community. The family's seemingly normal. Anytime this happens, it's a tragedy."Darrin Campbell had no criminal record or history of mental illness, she said. Yet in his final days alive, he exercised what appears to have been deliberate preparation for destroying his family.On Sunday, he bought a large number of fireworks and gas cans. On Tuesday, he made two purchases of gasoline from different stores. Early Wednesday morning, authorities were called to Avila as the Campbell home went up in flames. Inside, they found the members of the Campbell family dead in their bedrooms. Times staff writer Keeley Sheehan contributed to this report. Peter Jamison can be reached at [email protected] or (813) 226-3337.