More than a year before “The Blair Witch Project” hit theaters and became a cultural phenomenon, its central mystery had already gone viral.

According to the movie’s fledgling promotional website, which presented itself as a real investigative project, three film students — Heather, Mike and Josh — had ventured into the Maryland woods in 1994 to shoot a documentary and then disappeared. Their footage was recovered a year later, providing evidence to support a disturbing legend. The online message boards began to buzz, with questions about the story’s veracity.

The hype, intrigue and skepticism surrounding the account, fueled by the internet’s advent, grew through the movie’s premiere, in July 1999. What eventually emerged — a feature-length film made of spliced together scenes of shaky home video footage — made the demise of its three characters seem all the more authentic and terrifying.

Of course, the whole thing was fiction. But a lot of viewers didn’t know that going in.

“The prime directive we had was that the film had to look completely real,” said Eduardo Sánchez , who conceived and directed the entire thing with Daniel Myrick , from the manufactured legend to the film itself.