By Sandra Sobieraj Westfall
May 26, 2014 12:00 PM

The Campbells – Darrin, Kimberly and their two children, Colin and Megan – were the sort of family who played mini golf, reveled in their year-round passes to Disney World and went all out with Christmas decorations, once winning their Florida subdivision’s “Best- Decorated Holiday Home” contest a record four years in a row. “They were a perfect family,” Darrin’s mother, Mary Campbell, tells Mail Online. “They had everything to live for.”

But behind the regal entrance to the Campbells’ rented Tampa-area mansion, there were secrets seemingly worth dying – and killing – for. Contrary to Darrin Campbell’s image as the sunny, all-American dad, the business executive skirted the edge of financial and emotional ruin, a police source tells PEOPLE. It all exploded in a shocking murder-suicide May 7, police say, when four bodies were pulled from the charred carcass of the house owned by retired tennis pro James Blake. Now, while heartbroken relatives, school friends and neighbors in the gated community struggle to make sense of the barbaric and seemingly inexplicable crime, investigators see the deliberate act of a twisted mind. “We have evidence,” a police source tells PEOPLE, “that Darrin Campbell was very different behind closed doors.”

Some time before explosions sounded at 5:30 a.m., Darrin, according to police, fatally shot wife Kimberly, 51, son Colin, 18, and daughter Megan, 15. He arranged fireworks around their bodies, police say, before dousing everything with gasoline, igniting the bloody scene and then turning his .40 Sig Sauer on himself. Says a neighbor: “It’s unfathomable.”

And yet the puzzle pieces fit together chillingly. On May 4 Darrin, 49, bought several gas cans before stopping at Phantom Fireworks, where security cameras show him counting out more than $600 for what he told salesman Michael Muti was his “early Fourth of July” shopping list. “He was looking for the best deal and the best variety,” Muti recalls. “He was chatty.” Two days later, on the eve of the murders, he bought gas at two separate stores. While the police source says he is “150 percent” certain of Darrin’s hand in the crime, the why is more elusive.

Darrin, the youngest of seven children, called his mother every day, Mary Campbell, 79, told Mail Online. He met Kim when they both worked as aides in the Michigan state legislature. A full-time homemaker, Kim “was a really cool mom. Megan could tell her anything,” says Kayla Patterson, 16, a close friend of the high school freshman and honor student at Carrollwood Day School who also took classes at the All American Dance Factory.

Darrin, an M.B.A. who job-hopped as a finance whiz for various companies, was an attentive dad, says Patterson, who made Megan’s eyes roll with his “cheesy jokes and puns” and coached some of Colin’s baseball teams. Between father and son, however, things may not have been easygoing. Colin, a college-bound senior and baseball star, had “a slight touch of Asperger’s syndrome,” says William Bitting, a Los Angeles attorney who describes Darrin, his former colleague at Pabst Brewing Co., as “like a son” and Colin as “progressing, high-functioning.” The boy’s former Little League coach Paul Sickmon told The Tampa Tribune how Darrin, a former college athlete himself, would pitch to Colin in the batting cages “for hours upon hours.” That Norman Rockwell image now appears to have masked troubles: The police source tells PEOPLE the two came to fight over baseball, “and Colin told friends he and his dad didn’t get along.”

And even as Bitting insists Darrin was “even-keeled with zero signs of instability,” investigators see a picture emerging of financial and emotional stress. Unpaid taxes and liens over the past half-decade don’t wash with the Campbell family’s outward lifestyle: late-model cars and the lavish home rental at an estimated $5,500 a month, plus the Carrollwood tuition for both children that ran the family upward of $37,000 a year. The police source says his investigative team uncovered “several credit accounts with high balances” in Darrin’s name only. And, the source says, police have evidence that Darrin “had a mental or emotional breakdown” in the last couple of years.

A grieving Bitting allows, “Something just went wrong with Darrin, but we don’t yet know what.” Meanwhile, the mailbox outside the family’s burned-down home is stuffed with roses and cards. Says the neighbor: “The notes I saw were to Colin and Megan. They say, ‘We love you. Enjoy your time in Heaven.’ It’s heartbreaking.”