Fire investigators are trying to uncover what caused a fire at the Irving Paper mill in Saint John on Tuesday evening.

The fire started at about 5 p.m. on the roof of the mill's thermal mechanical pulping area and took about an hour and a half to contain, according to fire officials.

Part of the Irving mill, which is located on Bayside Drive, had to be evacuated during the fire.

Eric Garland, the Saint John district fire chief, said it was a challenge to battle the fire because it started high in the roof.

"Fire had spread to three different roofs that were all separate heights, making containment and extinguishment very manpower intensive. Fire crews worked diligently in conjunction with the emergency response team to extinguish the fire," Garland said.

Saint John fire crews remains at the mill for about five hours, tearing off roofing and extinguishing spot fires.

Garland said 26 firefighters were sent to the scene to contain the fire and only one injury has been reported.

"There were no firefighter injuries," Garland said.

"However, one Irving employee went to the hospital as a result of inhaling dry chemical extinguishment. This injury was not believed to be serious."

This is the second fire to break out at an Irving-owned facility in Saint John in the last week.

A small fire at the Irving tissue plant forced about 55 employees to temporarily evacuate the building on June 9.