Kelley Mack's death came as a "total surprise" to those close to her

A family source told TMZ that Kelley - whose real name was Kelly Klebenow - had experienced a "nice recovery" after undergoing treatment but she relapsed "out of nowhere", with the disease spreading through her body in recent weeks, leading to a "quick end".

The 9-1-1- actress spent an "immense" amount of time with her family and friends leading up to her death and was still working on voiceover jobs just a month ago.

A spokesperson for Kelley admitted her death had come as a "total surprise" because she had been auditioning for acting roles in early June, going up for parts of characters with mobility issues because she was using a wheelchair.

Although the actress had told her team she needed a break from auditioning in early July, her representative said: "We had no idea the turn it took and it was very unexpected,"

Kelley's sister Katherine announced her death earlier this week.

She posted on Kelley's Instagram account: "It is with indelible sadness that we are announcing the passing of our dear Kelley. Such a bright, fervent light has transitioned to the beyond, where we all eventually must go.

"Kelley passed peacefully on Saturday evening with her loving mother Kristen and steadfast aunt Karen present. Kelley has already come to many of her loved ones in the form of various butterflies [butterfly and heart emojis]. She will be missed by so many to depths that words cannot express.

"Kelley's Caringbridge link in her bio includes the 2-page formal announcement with information on her upcoming life celebration in Ohio on August 16th. She would want you all to know how much she loves you. And as her sister, I want you all to know how brave that tough SOB was, especially when she decided to make the leap to be reunited with God. I'm so f****** proud of her."

The actress - who also starred in a number of commercials - will make her final appearance as Ricky in upcoming movie Universal.

She is survived by her parents Kristen and Lindsay Klebenow, younger siblings Kathryn and Parker, grandparents Lois and Larry, and her boyfriend Logan Lanier.