“The Book of Mormon,” one of Broadway’s biggest juggernauts, will be forced to close for an unknown period of time after an electrical fire on Monday morning caused significant damage to the Eugene O’Neill Theater, where the musical has run for the last 15 years.

“The building is going to be out of service, pending repairs,” said David Simms, an assistant chief at the New York Fire Department. “There’s been substantial damage on the fourth floor, in the electrical room, which contains lighting equipment and hanging chandeliers.”

In a joint statement, “The Book of Mormon” and the theater’s owner, Ambassador Theater Group, said they would “take time to assess the extent of the damage, and any news of disruption to performances will be announced as soon as possible.”

“The Book of Mormon,” which won the Tony Award for best new musical when it opened in 2011, has been seen by nearly six million people on Broadway, where it has grossed $885 million over 5,640 performances. The show is also playing in London’s West End, and has touring companies in North America and Australia.

Simms said that the Fire Department was notified of a fire at the theater, which is on West 49th Street in Midtown Manhattan, at 10 a.m. on Monday, and that the fire was in an electrical room between the fourth and fifth floors. He said that more fire was then discovered between the fifth floor and the theater’s roof.

The three-alarm fire prompted a sizable response — 192 firefighters and emergency medical workers.

One firefighter, after sustaining minor injuries, had been transported to a hospital, Simms said, adding that “all theater personnel are accounted for.” Like many Broadway productions, “The Book of Mormon” is dark on Mondays.

“This was a deep-seated fire, involving a lot of electrical equipment,” Simms said. “It was very difficult to get to. There was a lot of catwalks and rigging in the theater.”

The fire was declared under control at 12:17 p.m.

Simms said that there had been “some water damage” and that the structure would need to be inspected by the city’s Department of Buildings. He said a theatergoer would not be able to see the damage because it was “in the back area.”

The cause of the fire was under investigation, Simms added.

Luis Geraldo, who works at a parking garage across the street from the theater, said he had noticed black smoke coming out of the building as the fire began.