Brook Windsor said she took the special photo as she was leaving the cathedral Monday afternoon -- about an hour before the landmark began to burn.
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Windsor believes the two were father and daughter based on their dynamic together, but said in a later tweet that it's possible they could have been a niece and uncle or family friends. She won't know for sure until they are found.
She said she almost approached the man to ask if he wanted the photo and now she wishes she had.
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Windsor has turned to social media for help and a little internet magic to track them down.
The storied Paris landmark caught fire in what is being treated as an accident. The massive fire burned for hours, sending up plumes of smoke visible across the city.
The Paris prosecutor said Tuesday morning there's no evidence of arson in the Notre Dame fire and that they're working on the assumption that the blaze was an accident.
It will likely be at least three years before members of the public are able to visit the Notre Dame Cathedral again, an official told ABC News.