Ryan Winderweedle, a spokesperson for the Gwinnett County Police Dept., shared the heartbreaking detail with us on Monday ... explaining Kidder's biological dad, Paul, told police he received a text from Kidder at around 4 PM -- just before he passed away.

Winderweedle said Paul then placed a 911 call, and Kidder was found dead from a gunshot at the family's home a short time later. Winderweedle said investigators discovered an envelope with a note inside that was left on a table in Kidder's bedroom.

Winderweedle added a Springfield Armory handgun was recovered at the scene.

Wendy Barlow -- Kidder's mom and Flair's ex-wife -- told TMZ Sports on Sunday she was "devastated and shocked" to learn of her son's passing.

Flair wrote in an Instagram post Sunday night that he, too, was torn apart by the death ... especially after having lost his own son, Reid, to a drug overdose in 2013.

"After Hearing The News Of Sebastian’s Passing," the WWE legend said, "I Can Only Imagine What Wendy @thewonderouswendy & Paul Are Going Through Having Lost A Son Approximately The Same Age In 2013. I Know The Pain Of Your Loss Will Be With You The Rest Of Your Life. I Still Every Day Of My Life Think About Reid As You Will Continue To Think About Sebastian."

Kidder, an up-and-coming musician, was just 24 years old.