ST. PAUL, Minnesota (AP) -- A 17-year-old girl who spent weeks looking for her missing dog unwrapped a box left on her doorstep and found the pet's severed head inside, authorities said.

Homicide investigators were looking into the case because of the "implied" terroristic threat, St. Paul Police Sgt. Jim Gray said. The Humane Society of the United States said Wednesday it was offering a reward of up to $2,500 for information leading to an arrest and conviction.

"This was extraordinarily heinous," said Dale Bartlett, the Humane Society's deputy manager for animal cruelty issues. "I deal with hundreds and hundreds of cruelty cases each year. When I read about this case, it took my breath away. It's horrible."

After Crystal Brown's 4-year-old Australian shepherd mix, Chevy, wandered away last month, she put up "missing" posters in her neighborhood and went door to door looking for him. She called the St. Paul animal shelter and rode the bus there several times.

"I felt empty," Crystal told the Star Tribune of Minneapolis. "I couldn't talk to anyone. He was my dog. It was just me and him. ... I told him everything and he never shared any of my secrets."

Two weeks ago, a gift-wrapped box was left at the house Crystal shares with her grandmother. The box had batteries on top, and a note that said "Congratulations Crystal. This side up. Batteries included."

Crystal opened the box and found her dog's head inside. The box also contained Valentine's Day candy.

Crystal screamed when she saw her dog's face.

"She was just hysterical," said Crystal's grandmother, Shirley Brown. "She was screaming. She said, 'Grandma, it's my dog's head!'

"I said, 'no it can't be!"'

Authorities say the case is an isolated incident and the suspect likely knew the family. A motive is unclear.

"This was so cruel," Crystal said. "This is one sick, twisted person."

She now has a new puppy, another Australian shepherd. She's named it Diesel.

"Hopefully, he'll be my best friend," Crystal said.