PARIS – An organist at Notre Dame Cathedral had just finished playing a psalm and the priest had started leading the gospel for the evening services when the fire alarm first sounded, startling those inside the historic church.

Johann Vexo, one of five organists who play at the Notre Dame in Paris. Courtesy Phillip Truckenbrod Concert Artists

Johann Vexo, one of five organists who play at Notre Dame in Paris, had no idea what was in store when he showed up for the Monday evening service.

In an interview with NBC News, conducted over FaceTime from his home in Nancy, about 175 miles east of Paris, the musician recalled the heartbreak he felt when he saw the beloved Parisian landmark engulfed in flames.

“It’s like seeing your own house burning,” he said, “it’s just unbearable.”

The evening Mass began at 6:15 p.m. local time (12.15 p.m. ET) and it wasn't long into the service when the fire alarm went off, causing confusion among church workers and visitors.

“Everybody was really surprised because it was the first time we had heard this alarm, so we didn’t really know what to do,” Vexo, 40, said.

It was then that people at the back of the cathedral — where the roof would later collapse — started to evacuate through the front entrance and the Mass had disbanded, he said.

Vexo, who plays at the cathedral weekly on Mondays or Tuesdays, first headed to the sacristy where he glanced at the alarm system, still in disbelief that the cathedral may actually be on fire.

“I was just thinking it was a mistake or a test,” he said. “I never thought that it was a true fire.”

In fact, he didn't learn that Notre Dame was in flames until he was back at his Paris home, a short distance from the church, and a colleague telephoned him. Images of the burning building were already reverberating around the world when Vexo descended into the streets to take in the horrifying spectacle.

It would take 400 firefighters some nine hours to extinguish the blaze that ripped through the roof, destroying a spire and threatening to raze the historic Gothic cathedral.

Paris was mourning its loss Tuesday when French President Emmanuel Macron vowed to rebuild the fire-ravaged cathedral within five years.

Vexo was only 14 when he began playing the organ, and he was appointed choir organist at Notre Dame at 25, according to his profile on concertartists.com.

Playing at Notre Dame was a “dream come true," he said.

“I think for every organist all around the world, it’s the best place, the most famous place in the world, just the top of the top,” Vexo said.

As for the state of the organs, he said it's not clear if the choir organ was damaged. But he said it had been filled with water as firefighters attempted to save the 18th century stalls where the canons sit on either side of the choir.

Vexo also said he received good information early Tuesday that the great organ — built in the 1730s by Francois Thierry — appeared to have been saved. But while it may look perfect from the outside, it will need a thorough check before the full state of the damage can be assessed.

The great organ has 8,000 pipes and, according to Vexo, it can be identified by its sound.

“The acoustic is absolutely stunning,” he added. “I think the great organ of Notre Dame is one of the only instruments that you can really recognize as soon as you hear it.”