HUNTSVILLE, Texas (KIAH) – A historic prison in Huntsville, Texas, caught fire early Friday morning, prompting the evacuation of hundreds of inmates.

The Texas State Penitentiary at Huntsville, known as the “Walls Unit,” caught fire at about 2:30 a.m., when heavy smoke was first reported. Crews arrived to find heavy fire coming from at least one of the seven housing units at the historic prison.

The main source of the flames could be seen coming from an old tower overlooking the facility near the parking lot.

The prison has a capacity of 1,700 inmates, and all 655 being held in the impacted unit were evacuated to another onsite location.

Officials said the fire was mostly in the attic, and the affected inmates had been held in an area just under the attic. No injuries have been reported.

While the fire at the state facility has been contained, there was no word on where it started or the cause. The fire marshal was investigating.

“This is pretty significant so it will take some time,” said Huntsville Fire Chief Greg Mathis.

The Walls Unit, which got its nickname because of its surrounding 15-foot brick wall, was established in 1849 as Texas’ first state prison, according to the Texas Department of Criminal Justice.