The cause of a fire last summer that destroyed the $9 million home of a Southampton Village real estate mogul whose clients include Polo Ralph Lauren, Donna Karan and Dolce & Gabbana, cannot be determined, a fire official said Thursday.

Village Fire Marshal Dean McNamara said in a telephone interview that after the roof caved in during the July 25 blaze at 50 Wyandanch Lane, there was so much damage that investigators could not determine with certainty what led to the blaze.

“There’s nothing we can say conclusively because we couldn’t do a complete investigation,” McNamara said. “We investigated all causes, but in the end it was classified as undetermined due to the damage and the collapse of the roof. When you have an entire roof collapse onto the second floor of an 8,340-square-foot home, you’re obviously going to have devastation throughout the home.”

The six-bedroom, six-bathroom home is owned by Lawrence and Marilyn Friedland. Lawrence Friedland owns Manhattan-based Friedland Properties, described on its website as the largest landlord on the tony “Gold Coast of Madison Avenue” shopping area.

Other prominent tenants include fashion designers Kate Spade, Valentino and Tom Ford.

Friedland could not be reached for comment Thursday.

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McNamara said officials believe the blaze likely started in the kitchen. He said the fire started just before 2:30 p.m. during a birthday party attended by about 30 people.

“It was reported from staff that they probably had a grease fire,” McNamara said. “There was fire in the crawlspace where the duct work went to.”

Two firefighters were treated for smoke inhalation.

Fire departments from Westhampton, Flanders, East Quogue, Hampton Bays, North Sea, Sag Harbor, Bridgehampton and East Hampton responded.