The horror began in January 1949 with a dripping sound — a faint trickle of distant water somewhere behind the walls of the Cottage City, Md., house.

Soon, a crucifix on the wall began to shake, as if someone or something had bumped into it. The dripping sound dissipated and a loud, unsettling scratching noise began beneath the floorboards in one of the bedrooms.

The scratching soon became a high-pitch squeak, similar to sneakers catching on a stone floor. The sound jumped to one of the beds and caused a violent shaking.

More disturbances followed, and within a few weeks, one of the house’s occupants — a 13-year-old boy known as “Roland Doe” — would end up apparently possessed, his body nearly torn apart by a spirit, leading to a torturous months-long struggle to cure him.

The incident remains one of the most famous cases of exorcism in American history and served as inspiration for “The Exorcist.” The case is revisited in the new book “Confrontation With Evil: An In-Depth Review of the 1949 Possession That Inspired ‘The Exorcist’ ” by Steven A. LaChance.

The movie was based on the bestselling 1971 novel by William Peter Blatty, who got the idea from a newspaper story about the Cottage City case. The article detailed Doe’s troubles, which were rumored to have begun when he played with a Ouija board.

Strange things started happening after Doe turned in for the night. His bed would shake and scratches would appear on his body that sometimes formed words. Vials of holy water placed nearby were said to fly off shelves.

In February of 1949, deep-red welts spelling out the word “LOUIS” appeared near the boy’s ribs, which his distraught mother took as a sign that she should take her son to St. Louis — her hometown.

There, a family member sought the aid of Father Raymond Bishop, a clerical professor at St. Louis University.

During the priest’s first blessing March 9, 1949, Doe’s bed began to shake and “zig-zag” scratches suddenly appeared on his body. A week later, the archbishop granted permission for a full exorcism.

“No one in that quiet neighborhood had a clue about the battle of good and evil that was about to take place in the quaint brick house,” LaChance writes.

The exorcism began at 10:30 p.m., conducted by Bishop and two other church officials. As the rites were read, Doe began shaking and violently banging his fists against the bed, much like Regan (played by Linda Blair), the young victim in “The Exorcist.” The word “HELL” appeared on his chest.

Weeks of almost nightly rites followed, as Roland spontaneously urinated, passed foul gas, laughed hysterically, spoke nonsensical Latin and spit at and cursed his attackers.

“The vile and filthy talk which followed makes anyone shudder,” Bishop wrote in his diary. (Blatty tried to obtain the journal at the time, but Bishop declined. To ease the exorcist’s anxiety, Blatty changed his protagonist from a boy to a girl.)

The movie’s most climactic moment, when the possessed Regan swivels her head 360 degrees, didn’t reportedly happen to Doe.

The boy’s haunting ended in April 1949, when he received a vision of a man “with flowing, wavy hair” holding a “fiery sword” and standing over a pit containing a devil, Bishop wrote in his diary. Doe suddenly declared, “He’s gone!”

According to published reports, the boy went on to lead a normal life.

But his story struck a cultural chord, tapping into the national anxiety that pervaded at the time.

“The demon was evil personified through the fears of a world that had seen genocide, the Kennedy assassination, the lies of Nixon, the atrocities of the Vietnam War, the growing generation gap, and of course the change of the family as well,” LaChance writes.

The novel and film brought the idea of demonic possession into the mainstream.

Some experts have since theorized that Roland wasn’t haunted but just mentally ill, although some subsequent residents of the St. Louis house where the exorcism took place have apparently reported unusual goings on.

The power of Christ compels you — to move.