It wasn’t clear late Monday what triggered a fire at Texas A&M’s Zachry Engineering Center, prompting more than 2,000 students, faculty and staff to be evacuated.

About 35 firefighters spent just under two hours Monday working on the blaze — which was in an electrical conduit, also known as a pipe chase. The location where the fire started was difficult to access, which is why it took so long to completely douse the flames, authorities said.

Still, firefighters said most of the damage was contained to the third floor and a small section of the roof.

No one was injured and the building is expected to reopen at 7 a.m. Tuesday, Texas A&M officials said, adding that afternoon and evening classes in the three-story building were canceled Monday.

Officials said crews responded to the automatic fire alarm set off around 2:15 p.m. and arrived on scene four minutes later to find smoke billowing from a stairwell.

At that point, the first responding unit called for a full structure response — three engines, a ladder truck, an ambulance and a command officer. By the time it was over with, eightCollege Station fire vehicles, including a ladder truck, and three from Bryan were dispatched to the scene, said Bart Humphreys, spokesperson for the College Station Fire Department.

The cause of the fire continued to be investigated late Monday, said Humphreys.

A&M officials said it was too early to estimate the cost of the damage.

The A&M Environmental Health and Safety Department will pick up the investigation with assistance from the University Police Department, officials said.

A&M’s emergency notification system, Code Maroon, was activated at 3:15 p.m., an hour after the alarm was tripped. It cautioned students and staff to avoid the area.