Suffolk police violated the civil rights of a Mount Sinai resident when they failed to search for him after he fled into the woods adjacent to his home in October, the man’s grieving father said in a lawsuit filed this week.

Casey Mendelsohn’s body was found by a deer hunter in Town of Brookhaven sump on Oct. 22, a week after his distraught father, Barry Mendelsohn, reported him missing, the lawsuit said. Police failed to investigate his 23-year-old son’s disappearance, insinuating that he was off somewhere doing drugs.

“I believe the police failure to investigate is motivated by discriminatory bias against the deceased based on Casey’s mental illness, which police completely knew about,” said the handwritten lawsuit, filed Wednesday in Suffolk County Supreme Court.

According to the lawsuit, Mendelsohn is seeking $100 million from the department.

Mendelsohn, who is representing himself in the litigation, said Suffolk authorities have not provided him with his son’s cause of death.

The complaint alleges that Suffolk police violated New York civil rights laws and the Equal Protection clause of the U.S. Constitution’s 14th Amendment.

Suffolk police declined to comment, saying the department cannot discuss pending legal actions.

According to the complaint, two Suffolk police officers came to the Mendelsohn home for an “unknown reason” on Oct. 8. One officer entered the home while the other stayed outside.

When Casey Mendelsohn saw the officer enter the home, he fled out the back door, through the backyard and into the nearby woods. “Officer (A) stated, ‘Why is Casey running, we are not here for him,’” the lawsuit said.

One officer said he would drive around to see if he could locate Casey, but he and his colleague did not enter the woods to search for him.”The officers never came by to state they could not locate Casey Mendelsohn,” the lawsuit said.

After Mendelsohn reported his son missing, a detective — described in the lawsuit as “condescending” — told him that he had spoken with a person named “Franky.”

“He insinuated that Casey Mendelsohn was doing drugs with other folks,” the lawsuit said of the detective.

Casey Mendelsohn’s body was found a week later by a hunter, the lawsuit said.

Michael O’Keeffe covers Suffolk County police and other Long Island law enforcement agencies. He is an award-winning journalist and the co-author of two books,”The Card” and “American Icon.”