Sydney Sweeney Mourns 'Euphoria' Co-Star Angus Cloud in Emotional Tribute: 'Hardest Thing' to Post

The 'Euphoria' actor died Monday after being found unresponsive by his mother in Oakland, California. He was 25.

Sydney Sweeney is remembering her Euphoria co-star, Angus Cloud, amid his untimely death.

Sweeney took to Instagram Tuesday to share an emotional tribute to Cloud, who died Monday at the age of 25, calling the young actor "an open soul."

"Angus you were an open soul, with the kindest heart, and you filled every room with laughter. This is the hardest thing ive ever had to post, and im struggling to find all the words," Sweeney wrote alongside some never-before-seen photos of her, Cloud and their Euphoria castmates. "You will be missed more than you know, but I'm so blessed to have known you in this lifetime, and I'm sure everyone who has ever met you feels the same."

She added, "This heartache is real and I wish we could've had one more hug and 711 run. All my love is with you."

Sweeney is just one of Cloud's co-stars from the hit HBO series to pay tribute to the late actor. In addition to Alexa Demie, Javon Walton, Cloud's co-star and on-screen brother and the show's creator, Sam Levinson, Euphoria star Zendaya also posted about Cloud's passing.

"Words are not enough to describe the infinite beauty that is Angus (Conor)," the actress wrote on Instagram. "I'm so grateful I got the chance to know him in this life, to call him a brother, to see his warm kind eyes and bright smile, or hear his infectious cackle of a laugh (I'm smiling now just thinking of it)."

"I know people use this expression often when talking about folks they love…'they could light up any room they entered' but boy let me tell you, he was the best at it," she continued. "I'd like to remember him that way. For all of the boundless light, love and joy he always managed to give us. I'll cherish every moment."

Zendaya finished her message with a message for Cloud's family: "My heart is with his mother and family at this time and please be kind and patient as grief looks different for everyone," she wrote. 

A source told ET Tuesday that the whole cast of Euphoria is in shock over Cloud's death.

"The whole Euphoria cast is very sad and shell-shocked over Angus' passing," the source shared. "They are leaning on each other and trying to be there for one another as they process this."

Meanwhile, in a statement to ET, an HBO spokesperson said, "We are incredibly saddened to learn of the passing of Angus Cloud. He was immensely talented and a beloved part of the HBO and Euphoria family. We extend our deepest condolences to his friends and family during this difficult time."

Cloud died on Monday at age 25, his family confirmed to ET. 

"It is with the heaviest heart that we had to say goodbye to an incredible human today. As an artist, a friend, a brother and a son, Angus was special to all of us in so many ways," the Cloud family statement read. "Last week he buried his father and intensely struggled with this loss. The only comfort we have is knowing Angus is now reunited with his dad, who was his best friend."

"Angus was open about his battle with mental health and we hope that his passing can be a reminder to others that they are not alone and should not fight this on their own in silence," the statement continued. "We hope the world remembers him for his humor, laughter and love for everyone. We ask for privacy at this time as we are still processing this devastating loss."

Additionally, a source close to Cloud's family told ET, "Angus had been battling severe suicidal thoughts after getting back from Ireland, where Angus and his family laid his father to rest. Angus was staying with his family as he tried to work through overcoming the grief."

Cloud's death is still currently under investigation by the Oakland Police Department, who in a statement to ET, said that Cloud was pronounced deceased at the scene. His cause of death is currently unknown.

ET has also learned that there are no funeral plans confirmed for Cloud at this time. 

For more on the actor's tragic death, see the video below.