Bill's bulldog also passed away in February.
The timing of all three dogs' deaths led the families to become suspicious.
Heidi Klum's husband Tom Kaulitz believes foul play might have led to the death of the couple's two dogs – Capper and Anton. (Instagram: Heidi Klum )
HEIDI KLUM SHARES REVEALING KISSING SELFIE ON FOURTH ANNIVERSARY WITH HUSBAND: ‘LOVE OF MY LIFE’
Tom Kaulitz's brother Bill Kaulitz is also suspicious about the death of his bulldog Stitch. All three dogs, seen here, died in February. (Instagram: Bill Kaulitz)
"Stitch is also at an autopsy, where we hope to learn a few more things," Bill said in German on the podcast, per People magazine. "Sadly, we also had to think about whether it might be the case that everything is connected."
Tom added, "It's so sad that you have to think that there is someone who wants to harm you so much that he kills your animals. Is there a lunatic out there?"
The two questioned if the dogs had been poisoned.
Bill explained that his 4-year-old bulldog has never had any health issues.
"He was never sick, never had to go to the doctor," the German singer-songwriter explained. "And then, one Sunday, he suddenly collapsed and fell over. I was in shock."
Bill Kaulitz's dog Stitch poses for a photo. (Instagram: Bill Kaulitz)
Tom honored his dog Anton in an Instagram post after his death.
"I will never understand why life took you from me so early, so unexpectedly, so sudden, only days after we lost our Capper, when you were only 4 years old and we both still had so many plans together," he wrote. "My heart is broken and miss you more than words could ever say."
"Thank u for being my home, my fearless and strong protector, for making me laugh and feel joy ever day [sic] looking at your little face," he continued. "I love u so much and will keep u in my heart forever!"
Heidi Klum and Tom Kaulitz take a selfie with their dog Anton. (Instagram: Heidi Klum)
The supermodel shared a tribute on Instagram to the couple's dog Capper.
"Forever Loved Forever in our hearts," Klum wrote.
She later shared a series of photos of a day spent in Palm Springs with Tom and their dog Anton.
"Such a beautiful Day Today," she captioned the photos. "We did not know it would be our last."