An American tourist who witnessed Monday's catastrophic fire at Notre Dame Cathedral in Paris recalled late Monday that the smoke from the blaze initially resembled a chimney fire before turning an ominous shade of dark black as flames spread and devoured the interior of the centuries-old sanctuary.

David Almacy, a former White House Internet director under George W. Bush, told Fox News that he, his wife, and two daughters had finished a tour of the smaller Saint-Chappelle chapel, not far from Notre Dame, and decided to walk over to the cathedral at around 6:30 p.m. local time.

"As we were walking over that way, we started to smell smoke and I think it was my younger daughter who looked up and said, 'What's that?'" Almacy said. "Initially, it [the smoke] looked like it was coming from behind the building."

SHOCKING PHOTOS: NOTRE DAME CATHEDRAL CATCHES FIRE

The Almacys had been in line to enter Notre Dame earlier in the day but opted to postpone their visit when the time slots for guided tours of the building filled up.

"We thought, 'We'll come back tomorrow,'" said Almacy, who arrived back to Notre Dame with his family shortly after the cathedral's 6:45 p.m. closing time and first noticed smoke from the blaze at approximately 6:50 p.m.

"Initially, there was no frenzy," said Almacy, who added that he did not see any fire engines for approximately 20 to 30 minutes. "There were a lot of sirens and a lot of police vehicles."

The situation became more serious at around 7:30 p.m. when Almacy described seeing "flames shooting out" of the building as "the smoke turned orange, then green, then dark black."

"We didn't know, was it an attack? Was it an accident?" He said. "We decided to move away and let the first responders and law enforcement do their work. Plus, we were having trouble breathing, our lungs were starting to hurt a little bit."

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As they walked, the Almacys passed hundreds of bystanders who had gathered to watch the horror from the bridges across the Seine River. They did not see Notre Dame's centuries-old roof collapse in the gathering darkness.

"This building took 200 years to build and it was destroyed by fire in less than two hours," Almacy said Monday evening. "It's a reminder to not take the things that we have for granted."