It's been less than two hours since reports of Mac Miller's death emerged, but already, celebrities have begun to mourn the rapper online.

"Unreal. So sad. RIP Mac Miller," tweeted Shawn Mendes simply.

Chance the Rapper, who toured with Miller in 2013, shared a more personal tribute.

"I dont know what to say Mac Miller took me on my second tour ever. But beyond helping me launch my career he was one of the sweetest guys I ever knew," he wrote. "Great man. I loved him for real. Im completely broken. God bless him."

He continued: "Its so crazy cause earl literally hit me up this morning him and vince were real friebds I met at a pivotal time in my life through mac. This s--t hurts so bad. If you love someone [make] sure u tell em."

Miller, who was 26, died on Friday of an apparent overdose, Variety confirmed. He was reportedly found in his Studio City, California home and pronounced dead at the scene.

Though details at this time remain murky, Miller has been open about his struggle with substance abuse in the past. In May, he was  charged with a DUI  after crashing his car into a pole in Los Angeles.

"I made a stupid mistake,” he later said of the incident during a Beats 1 interview. “I’m a human being. Like, [I] drove home drunk. But it was the best thing that could have happened. … I needed that. I needed to run into that light pole and literally, like, have the whole thing stop.”

See more celebrity reactions below.

If you or someone you know is seeking help for substance use, call the SAMHSA National Helpline at 1-800-662-HELP(4357).