Kate Spade wrote a note telling her daughter it wasn't her fault before she took her own life, according to reports.
The designer, who built an empire from her famous handbags, was reportedly found by a housekeeper in New York apartment around 11am (1am AEST).
She was 55 years old.
Read more: • Handbag designer Kate Spade found dead at 55
Police said Spade wrote a note, which sources claim was addressed to her 13-year-old daughter, Frances Beatrix Spade.
"At this point, there was a note left. The contents of that note, as well as the physical state of the apartment and the comments of the witness, lend to the credibility that it is an apparent suicide," NYPD Chief of Detectives Dermot Shea told reporters.
TMZ reports that the note was found nearby, and according to law enforcement sources, it read: "Bea — I have always loved you. This is not your fault. Ask Daddy!"
It's understood the fashion mogul's husband, Andy, was also at their Upper East Side home when her body was discovered, while their teenage daughter was at school.
The couple appeared to be having relationship problems, according to sources.
"It was over family problems … in her relationship," a police source told The New York Post.
She was married to Andy Spade, the brother of actor David Spade, and was also the aunt of House of Cards actor Rachel Brosnahan.
Ms Spade launched Kate Spade Handbags in 1994, which went on to become a global empire, attracting high-profile customers including Kate Middleton.
Spade was famous for her colourful, cheerful designs, which often incorporated eye-catching prints, bright shades and embellishments. She took the game-changing step of putting the brand label on the outside of the bag, rather than the inside.
Despite its success, Ms Spade sold her company in 2007 to spend more time with her daughter, but re-entered the industry in 2016 with luxury footwear and accessories brand Frances Valentine.
There were more than 140 Kate Spade New York outlets in the US alone, and more than 175 worldwide - an empire built on Spade's style.
"Every Kate Spade bag was colourful, bright and a little quirky," said news.com.au's fashion commentator Melissa Hoyer.
"There is no question that anything with the Spade label on it made people smile.
"Ms Spade seamlessly segued from the power dressing of the 80s into the androgynous 90s, where there was still a market for women wanting accessories that did have a fun and quirky sense of style."
The Kate Spade company released a statement expressing its condolences.
"Although Kate has not been affiliated with the brand for more than a decade, she and her husband and creative partner, Andy, were the founders of our beloved brand. Kate will be dearly missed. Our thoughts are with Andy and the entire Spade family at this time," Julia Curry, a senior manager for the brand, said in the statement.
Celebrities across the fashion world and beyond quickly took to Twitter to express their sympathy.
"Kate Spade's tragic passing is a painful reminder that we never truly know another's pain or the burden they carry," Ivanka Trump tweeted just after 1pm. "If you are struggling with depression and contemplating suicide, please, please seek help."
Chelsea Clinton also shared her condolences, saying she was given her first Kate Spade bag in college by her grandmother.
"I still have it. Holding Kate's family, friends and loved ones in my heart," Clinton tweeted.
The Council of Fashion Designers of America expressed "our most sincere condolences to the family."
"She was a great talent who had an immeasurable impact on American fashion and the way the world viewed American accessories," the group said in a statement.
Friends and family close to Ms Spade were also shocked by her untimely death.
The designer's niece Whitney Pozgay, who runs the fashion brand WHIT, wept when reached by phone, saying she'd only just heard about her aunt's death.
"She was wonderful," she said. "We have no comment at this time. We are just learning about it through media reports."
Spade's neighbour Shelly Bitt, 21, who used to work for the designer in California, couldn't believe the news.
"I just saw her here a week-and-a-half ago. She didn't say anything, she just smiled and waved. This is horrible," Bitt said. "She was a nice woman, always had a smile on her face."
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youth services: (06) 3555 906
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• The Word
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• CASPER Suicide Prevention
If it is an emergency and you feel like you or someone else is at risk, call 111.