An owner of a Long Island animal sanctuary and at least 100 of his cats were killed in a suspicious fire Monday morning that police are investigating for arson.

Authorities said the blaze started around 7:15 a.m. at the Happy Cat animal sanctuary in Medford. The firefighters had the Dourland Road fire under control by 8:35 a.m., but not before the sanctuary had burned to rubble.

Suffolk County police confirmed sanctuary owner Christopher Arsenault, 65, was killed in the inferno — believed to have died trying to rescue the felines from the home.

Officials said at least 300 cats lived on the premises.

Detectives from the homicide and arson squads are investigating the cause of the blaze, police confirmed.

“It’s too early in the investigation to determine any cause,” William Doherty, the Suffolk County police chief of detectives, said in a statement.

John DeBacker, a Long Island animal rights activist, claimed in a post on his Facebook page that Arsenault was being “harassed” for months and speculated that the fire could be connected.

“Despite being harassed for months, he continued to fight for the cats,” the post read, “and I truly hope everyone can screenshot posts from one of the groups that has been harassing him incase arson is connected.”

DeBacker did not respond to The Post’s request for comment.

Roy Gross, chief of the Suffolk County SPCA, said the ordeal Monday was “tragic.”

“It’s a horrible situation,” he told The Post.

The sanctuary, identified as Happy Cat Inc. in Medford, is a nonprofit “safe haven for cats in need,” according to property records.

Gross said Arsenault started the sanctuary in 2006, after losing his son in a motorcycle accident, and had been planning to move his practice upstate.

The SPCA and other rescue groups were on site Monday helping corral the remaining cats in an effort to find them new homes and provide medical attention, according to Gross.

The investigation into the cause of the fire is ongoing.