Firefighters Tuesday morning finally cleared the scene of a smoldering warehouse fire in northwest Atlanta, more than 36 hours after the three-alarm blaze began on Sunday night.

No one was seriously injured in the fire, though one firefighter had to be treated at a hospital for heat exposure. Authorities were still trying to determine what caused the fire at the Atlanta Paper Recycling warehouse on Mayson Turner Road, Atlanta fire Sgt. Bobby Stewart said late Tuesday morning.

The flames were extinguished by Monday morning, but firefighters remained at the site throughout the afternoon and Monday night, keeping an eye on smoldering bales of paper and plastics.

More than 100 firefighters responded to the three-alarm blaze, which began around 8:30 p.m. Sunday.

One of them was injured. The firefighter, whose name has not been made public, was treated at a hospital for dehydration and heat exhaustion and was released by Monday afternoon, Stewart said.

The fire grew quickly.

Capt. Joe Hussey said the first responding firefighters "actually did go inside with lines, saw what they had, and realized it was too big to fight with hand lines. They got the ladder truck up and sprayed the big water in through the roof."

The rear third of the building collapsed, causing additional issues for fire crews, Hussey said.

In a building the size of the large warehouse, "collapse is your biggest concern," Hussey said.

"You've got to stay outside the collapse zone," he said. "Falling brick hurts when it hits you, especially if it hits you in the noggin'."

Keyona Massey, who lives across the street from the warehouse, told the AJC Monday that it appeared to her that the fire possibly started in the woods adjacent to the building.

"Me and my daughter were coming out to go to the store, and she was like, ‘Mom, there’s a fire over there,'" Massey said. "I stood up there on a chair and said, ‘It sure is a fire. Give me my phone and let me call 911.'"

Massey said the fire was small when she made the call to 911, but "it looked like it was getting ready to get out of hand, and it did, too. After that, it just took off. It got out of hand real bad."

Massey said she could feel the heat from the fire, which she said "was looking like a tornado; it was twisting around and everything."

--Staff writers Ty Tagami and Alexis Stevens contributed to this article.