The 1984 murders of Brenda Lafferty and her 15-month-old daughter Erica by two of her husband's brothers shook their insular community—and put a spotlight on the dark side of Mormon fundamentalism.

Walking into the kitchen of the apartment he shared with his young family in American Fork, Utah, at 8 p.m., he found his 24-year-old wife, Brenda Lafferty, lying in a pool of blood.

The phone cord had been ripped out of the wall, so he headed to the bedroom to call 911 from there. Passing their 15-month-old daughter Erica's room, he saw that the toddler and the blankets in her crib were also covered in red.

The bedroom phone not working, either, Allen had to go to a neighbor's place to call the police, then he returned to Brenda's side and prayed.

"And then as I stood," he testified in court years later, "I surveyed the situation a little more, and realized that there had been a grim struggle."

Jon Krakauer detailed what Brenda struggled against—the killers she fought on the day she and her child died, and the destructive forces coming from within the Lafferty family—in his 2003

book Under the Banner of Heaven, the basis for the FX on Hulu limited series premiering April 28, starring Daisy Edgar-Jones as Brenda and Andrew Garfield as the lead detective whose own faith is shaken to the core as he uncovers the truth.

"Reading what she wrote in her journals," Betty recalled, "my dad started thinking, 'Why didn't I do something to save her? Why didn't I get her out of there?' As her father, he thought he should have been able to protect her somehow, but he couldn't."

Under the Banner of Heaven premieres Thursday, April 28, on FX on Hulu.