Authorities said no one was hurt in the blaze, which took place at Shepard Middle School on Dakota Street, a few blocks south of the N.C. Central University campus.

The county fire marshal's office is investigating the cause of the blaze, but Harold Boyd, a city fire investigator who was at the scene, said there were no immediate indications it was started deliberately.

"I don't know of anything suspicious," he said.

A passer-by called 911 to report the blaze at 1:38 a.m. Friday, according to Boyd.

"It was gutted," he said. "The roof and most of the walls were gone."

The modular classrooms built of wood, have asphalt shingle roofs and burn easily, according to Boyd.

"It's pretty much an open interior, there's not so much to slow it down," he said.

Durham Public Schools spokesman Michael Yarbrough said the classroom housed a sixth-grade class with 25 students by day. They were moved to the media center for classes Friday.

"It did not throw a kink in their day," Yarbrough said.

A replacement mobile unit will be in place by next week, Yarbrough said, possibly as soon as Monday.

Plans were nailed across the tan structure's badly warped door Friday, the roof and parts of the wall missing, the interior open to the sky.

"I heard trucks and things, but I didn't get up to check it out," said Ennis Lee, who has lived on Anacosta Street, just across from the cluster of mobile classrooms, for more than 40 years. "By the time I came out it was all over."

Lee used to work as a custodian at Shepard, but said he didn't remember any fires during his time there.

"It might have been that air conditioner," he said, noting black plastic covering holes the fire had burned in the wall ? including the spot where the air-conditioning unit had once sat.