Damage is now expected to top $100,000 after vandals set a fire inside a historic Westchester County lighthouse Friday night into Saturday, authorities said.

One year ago, the 1883 Tarrytown Lighthouse had just completed a $3 million renovation as the beloved landmark got extensive repairs, but now the boarded-up windows outside hint at the extensive damage inside.

Surveillance video leads police to believe at least four vandals got inside. County police are working aggressively to find them.

The fire left heavy soot on a door, and the people pushing it open left handprints. All of that's being analyzed.

"They literally set the main floor on fire"

County Executive Ken Jenkins said everything will be repaired, but the focus now is on finding who did this and why.

"I'm disappointed, discouraged. Very heartbreaking that this has happened," he said.

Westchester Parks Commissioner Kathy O'Connor says facilities see occasional vandalism.

"This is a whole different ballgame because they literally set the main floor on fire," she said.

The vandalism is now a sad chapter in the long history of the landmark. 

"I just thought it was so despicable. I can't even tell you how – I was shocked and really angry. It was just awful," Tarrytown resident Cynthia Semsel said.

"I just can't believe that someone would have the nerve to damage it in any way because it's so much a part of the history of this region," retired teacher Mara Cohen said.

It's not clear when this piece of history on the Hudson will reopen for tours.