The evening of Jan. 5 was everything Charlie and Katie Heitzig loved about their bubbly daughter, 12-year-old Grace. She was so excited about planning a fun party for her upcoming birthday — she was certain one friend liked crushed Oreos as a sundae topping — that she had to be ordered to bed.

The next morning was their nightmare.

Katie went into her daughter's bedroom around 6:30 a.m. to wake Grace for school and found her wrapped under her blanket and lifeless. Their smart, athletic, caring daughter was dead — and a week later nobody is sure why.

"Twelve-year-old kids just don't die suddenly like that," said her father.

While authorities continue to investigate how the Maple Grove seventh-grader died — beyond the conclusion that it was from as-yet undetermined natural causes — her family and friends are coming to grips with their abrupt loss.

Purple was Grace's favorite color, and it could be found on mailboxes and fences across Maple Grove last week.

The symbolic rock on the high school grounds was painted purple and covered with purple flowers. Her middle school classmates plan to wear purple on Friday, which would have been her 13th birthday.

Grace had an infectious energy and was "a gamer" by nature, said her mother.

She would try to play the piano or the flute, or run for student council, or make a chocolate lava cake, or play Jenga or Payday or whatever game was available until her parents and older sister, Ellie, couldn't take playing another round.

Grace tried Nordic skiing for the first time this winter, and wasn't very natural at it, but kept practicing and following the advice of her high school team captains.

Swimming was one of her favorite activities — a skill she perfected in the summers as she lapped around the island at her family's lake cabin in Ely.

Grace joined the Maple Grove High School junior varsity team in the fall, and earned respect for her willingness to try new strokes and different events.

"Her coach called her Iron Girl," said her father, "because she just would try. She had a stubborn determination — a willfulness."

Other than a seizure she unexpectedly suffered in the summer, Grace hadn't had any significant health problems, her parents said. Nor was she sick or suffering from flu-like symptoms in recent days, they added.

Her parents are left wondering whether she suffered another seizure that proved fatal. But her mother is trying hard not to fixate on that question.

"I can't sit here and dwell on it, because I'll never have an answer," she said. "The kids, though, they don't understand it. My [older] daughter is very logical. She's a science-math girl. You plug a formula in and make it happen. She's really trying to plug something in and the answer will come out and she'll feel good. But this is so gray."

The mischief in Grace's life is now being remembered alongside the successes. Her uncle turned on his phone two weeks ago to find that Grace had taken 100 selfies with it.

Her parents recently discovered that she had been sneaking in television on her iPod when she was supposed to be studying or sleeping in her room. The history on the ABC television app was filled with Christmas movies and specials.

Charlie Heitzig said he will miss his daughter's spirit and compassion.

After birthday plans to pack food for charity fell through, she approached her father with a new plan for a party her friends would enjoy.

"Daddy, can I tell you about my birthday party?" he recalled her saying. "She still called me daddy," he said.

Services have been held.

Jeremy Olson • 612-673-7744