Darren Criss is mourning the death of his brother Charles "Chuck" Criss. He was 36.

On Wednesday (March 2), Darren shared an emotional Instagram post with photos of him and Charles over the years along with a lengthy letter addressing Chuck's passing. The caption revealed that he died sometime in February.

Darren expressed that his passing “breaks [his] heart beyond measure” and that it was “a colossal shock” to his family.

"His loss leaves behind a debilitating fracture in the lives of his mother, his brother, his three small children and their respective mothers. I have spent what already feels like a small eternity trying to wrap my head around it, something I suspect I’ll be attempting to do for the rest of my life," Darren wrote.

Darren explained that he had to meditate to decide how much information regarding Chuck's passing he was willing to share with the public. While it is an "extraordinarily sensitive and private matter," he also noted that "vagueness can lead to confusion, and provide ideas far worse than the truth."

Darren said that the brothers' strong relationship began "right out of the womb."

"He was my instant, ready-made best friend. And from that moment on, we absolutely loved being together," Darren shared.

Over the years, the two have been "unwaveringly supportive audience members" for each other's endeavors. The brothers even collaborated in 2017, forming a band called Computer Games.

Darren explained that the last several years were "increasingly difficult" for Chuck as "he struggled to find stability during an unfortunate rough patch in his life." Chuck told his family that everything was fine but now his loved ones know that he "clearly had a severe depression welling up in him for some time, a depression that was only worsened by a lifelong struggle he had with expressing his feelings — a dangerous combination truly outmatched by his all-too-incredible ability to conceal it."

Darren continued, "We will never know just how long he was fighting this war within himself, but last week, it consumed him fully, and he succumbed by taking his own life."

See the letter in full, below.

Darren encouraged anyone who is struggling with mental health to seek help and expressed that "there is simply no sense, nay, compassion or decency, to try to find blame in any direction other than the innermost tragedy of untreated illness.”

Almost two years ago, their father, Charles William Criss Jr., died following the diagnosis of a rare heart condition.

If you or someone you know needs help, please contact the National Suicide Prevention Lifeline at 1 800-273-TALK (8255).

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