Robert Downey Jr. is in mourning.

The "Avengers: Endgame" star, 56, took to Instagram on Thursday to announce the death of his longtime assistant, Jimmy Rich, who, according to the actor, passed away in a car accident.

In his post, Downey shared a trio of photos of Rich.

"This is not news. This is a terrible and shocking tragedy," he wrote in the caption. "Jimmy Rich was in a fatal car accident at approx. 8pm yesterday. He was a brother, my right hand man, an uncle to our kids and beloved by all who experienced his singular character and wit."

The actor sent sympathies to Rich's "relatives, friends, co-workers, and all the fans who knew him as the man who supported every step of my recovery, life and career."

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Downey famously faced addiction throughout his life, having been exposed to drugs at a very early age.

Robert Downey Jr. said on Instagram that his longtime assistant and friend Jimmy Rich has died.  (Photo by Axelle/Bauer-Griffin/FilmMagic)

Robert Downey Jr. said on Instagram that his longtime assistant and friend Jimmy Rich has died.  (Photo by Axelle/Bauer-Griffin/FilmMagic)

"Again, my condolences to his beautiful family, and the legacy of hope and redemption his life will continue to represent," he concluded. "Peace Be Unto You."

Being such an important part of Downey's life, it stands to reason that Rich could have crossed paths with some of the actor's Marvel co-stars, who shared their condolences as well.

"Wow. Brother," wrote Jeremy Renner in the comments. "What a devastating loss."

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"Oh no! I’m so sorry man. Im praying [for] his family, you and yours," commented Chris Pratt. "That sucks so bad. I’m so sorry to hear that. He was awesome. Such a tragedy."

Added Mark Ruffalo: "Such a tragedy. Such a good man. I have known him almost as long as I have known you, Robert. Breaks my heart to have Jimmy go over to the other world. You will be missed Jimmy. Thanks for always being real, always being there to lend a hand and showing the way of light forward to so many people. Another sad day."

Chris Pratt, Mark Ruffalo, Chris Evans

Marvel stars Chris Pratt, Mark Ruffalo and Chris Evans also posted condolences. (Getty Images)

Rich received a tribute from Chris Evans on his page as well.

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Alongside a photo of himself and his late pal, Evans wrote: "I don’t have the words. Jimmy Rich was unlike anyone else. He left an indelible impression on everyone he met. No one more so than my brother @robertdowneyjr They were inseparable."

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"I’m sending all my love to Jimmy’s friends and family," the "Captain America" star, 39, continued. "Be sure to hug your loved ones tightly and please take the time today to show Downey how many people love him and are here for him during this incredibly dark time."