American actress Michelle Trachtenberg mysteriously died this week at the age of 39, and there has been an outpouring of grief and tributes from many of her heartbroken friends and fans.

That includes a member of the British Royal Family, Princess Eugenie, who mourned the loss of her pal in a social media post on Thursday, the New York Post's Page Six reported.

It is unclear how or when the British princess and American actress became close friends, but the news outlet shared several photos of the two young women hanging out together over the years.

A special goodbye for a "special friend"

In an Instagram Story posted on Thursday that featured a black and white photo of the two women hugging and smiling, Princess Eugenie wrote, "Goodbye special friend. You are so loved and missed. Thank you for so many memories of laughter and fun."

"You were so thoughtful and generous and soooo funny," she continued of Trachtenberg. "My favourite was listening to you order food, always done with precision so you got exactly what we all wanted."

"I’m thinking of you up there friend, of your spirit that was so bright and your joy you brought so many," Eugenie added.

Cause and manner of death left "undetermined"

It is unclear how or why Trachtenberg died, but Page Six reported that the actress, who was found dead in her New York City apartment by her mother on Wednesday, had previously dealt with unspecified health issues and had recently received a liver transplant.

Unfortunately, the world may never know the answer to those questions, as Trachtenberg's family declined to have an autopsy performed on her body, according to USA Today and other media outlets.

As such, the Chief Medical Examiner for New York City listed the cause and manner of her death to be "undetermined." That said, the NYPD is conducting an ongoing investigation of the woman's demise, though no foul play is suspected.

Meanwhile, Trachtenberg's family has remained tightlipped and, aside from confirming Michelle's death with "great sadness" in a statement shared through a representative, said only that "The family requests privacy for their loss. There are no further details at this time."

Decades-long acting career

According to Trachtenberg's IMDb page, the actress was credited with appearances in 63 films and TV series between 1991 and 2023, with her first major recurring role coming in the mid-90s Nickelodeon series "The Adventures of Pete & Pete."

Her fame only grew with starring roles in TV series like "Meego," "Buffy the Vampire Slayer," "Mercy," and "Gossip Girl," among others.

Trachtenberg also starred in several popular films as a child and as an adult, including "Harriet the Spy" and "Ice Princess," plus dozens more.

Her decades-long acting career also included some voiceover work for animated characters plus appearances in music videos and commercials.