“Saturday Night Live” writer Jimmy Fowlie announced a tragic development in the search for his missing sister … saying that she’s “no longer alive.”

Christina Lynn Downer had been missing for months … and Jimmy’s been posting pleas on social media asking for help finding her.

But on Wednesday, he said the LAPD has told his family that the search for Christina is no longer a missing person’s case … but now a homicide investigation.

He wrote on Instagram … “Our prayers for her to be found safely have transformed into prayers for the truth to be revealed and for those responsible to be held accountable.”

The ‘SNL’ writer says he believes her phone and social media were “compromised” in the weeks leading up to her disappearance late last year … and whoever has her phone “used it to hide the fact she was gone, to ask for money, and to create a false narrative that she was going ‘off the grid.'”

Jimmy went on to say that he thinks whoever’s responsible wants to “erase her in every way possible,” so he’s determined to spread her story.

He wrote … “She was a beautiful person who matters in this world, especially to me.”

He urged anyone who knows anything about her death to come forward … and said the best way anyone can support him right now is by telling his sister’s story.

According to an LAPD post from last December … Christina was last contacted on December 10, when a friend texted her, and her last known location was the Koreatown neighborhood of L.A.

In the caption of his post, Jimmy said he’d be taking some time off social media while he attempts to process his “overwhelming feelings” about the loss of his sister.