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The 31-year-old actor was found dead Saturday afternoon in a Vancouver, Canada hotel.
Monteith played heartthrob Finn Hudson on the series since 2009.
Celebrities and "Glee" co-stars took to Twitter to react to the star's passing.
Many were shocked.
Co-star reactions along with those who appeared and worked on the show:
The guy I shot my first acting scene with. The nicest guy I've met. Can't believe it.
— Damian Mc Ginty (@damianmcginty) July 14, 2013
What I will remember about Cory Monteith is that he was talented, funny and absolutely kindhearted. We lost a gentleman. RIP
— Cheyenne Jackson (@cheyennejackson) July 14, 2013
RIP CoryMontieth. You will be loved. Always.
— Kristin Chenoweth (@KChenoweth) July 14, 2013
I will always cherish you in my heart. Thank you for so many wonderful days. I love you CM ... pic.twitter.com/HW5me8dmLo
— Adam Shankman (@adammshankman) July 14, 2013
Dot-Marie Jones played Hudson's football coach on "Glee":
I HAVE NO WORDS! MY HEART IS BROKEN CORY WAS NOT ONLY A HELL OF A FRIEND HE WAS ONE AMAZING MEN THAT I WILL HOLD CLOSE TO MY HEART FOREVER
— Dot-Marie Jones (@dotmariejones) July 14, 2013
I AM BLESSED TO HAVE WORKED WITH HIM AND LOVE HIM SO MUCH! MY HEART IS WITH HID FAMILY AND OUR WHOLE GLEE FAMILY! I LOVE YOU ALL!
— Dot-Marie Jones (@dotmariejones) July 14, 2013
Vocal producer and recording engineer on "Glee," Alex Anders:
Heartbroken by the loss of my friend @CoryMonteith Praying for his family and loved ones. Such a good hearted man that will be sorely missed
— Alex Anders (@alxanders) July 14, 2013
"Glee" co-star Mark Salling since deleted a Tweet "no" in response to an article claiming Monteith died of a drug overdose.
His girlfriend and co-star, actress Lea Michele hasn't made a statement, but Hollywood Life reports she is "absolutely hysterical and completely inconsolable."
Celebrities responded with prayers and shock at the news.
Please tell me it's not true
— Kaley Cuoco (@KaleyCuoco) July 14, 2013
#CoryMonteith no words. Prayers. Thoughts w his family. This is terrible. Can't speak.
— Kaley Cuoco (@KaleyCuoco) July 14, 2013
Way too soon. A very nice & good guy. RT @joshgad: RIP Cory Monteith. Gone way too soon.
— Joel McHale (@joelmchale) July 14, 2013
This is such sad news about @CoryMonteith - what an absolutely tragic loss of a very talented young man.
— zooey deschanel (@ZooeyDeschanel) July 14, 2013
Speechless. And for the worst reason.
— Taylor Swift (@taylorswift13) July 14, 2013
Just heard about the
— Kim Kardashian (@KimKardashian) July 14, 2013death of Cory Monteith. So sad. Prayers 4 his family. Praying 4 Lea too! Words cant describe what they must be feeling
With ever ounce of my body praying about Cory and his family and his friends. God bless all involved. Heart broken.
— Lucy Hale (@lucyhale) July 14, 2013
RIP Cory Monteith. My thoughts and prayers are with you and your loved ones.
— Eric Stonestreet (@ericstonestreet) July 14, 2013
I am so sad to hear about Cory Monteith. He was a lovely person with a ton of talent. My heart goes out to his family and friends. RIP Cory.
— Jesse Tyler Ferguson (@jessetyler) July 14, 2013
Cory Monteith, may your spirit be at peace, and may you fly with the angels... Heartbreaking, my prayers are with all of his loved ones!
— Rihanna (@rihanna) July 14, 2013
Rest in peace Cory. My heart goes out to his family and loved ones. Our prayers are with you.
— Katy Perry (@katyperry) July 14, 2013
Not at all what I expected to read when I woke up in London. Damn. Stay strong gleeks. No one deserves to go that young.
— Matt Prokop (@mattpro13) July 14, 2013
How sad to read about Cory Monteith passing away. What a shame. He was a lovely, kind, talented guy and will be missed.
— Neil Patrick Harris (@ActuallyNPH) July 14, 2013
Beautiful man! Why did you leave us so soon? Rest with the angels, Cory Monteith.
— Alyssa Milano (@Alyssa_Milano) July 14, 2013
I just heard about Cory Monteith.. Rest in peace, everyone close to him and affected by this loss you're in my prayers.
— Chloë Grace Moretz (@ChloeGMoretz) July 14, 2013
So heartbreaking to hear about Cory Monteith...my deepest condolences to his friends and family.
— Ellen Page (@EllenPage) July 14, 2013
I'm gutted. Shocked and saddened to hear the news, my heart goes out to Cory's family and friends. Lets respect their space as they grieve.
— Nina Dobrev (@ninadobrev) July 14, 2013
So sad to hear the news about Cory Monteith. Praying for his family and friends.
— Nick Jonas (@nickjonas) July 14, 2013