WATERVILLE — A high-rise apartment building on Elm Street was evacuated Thursday after a fire broke out about 4 p.m. in a seventh-floor apartment.

Dozens of residents assembled outside Elm Towers as an alarm blared. Firefighters from Waterville and Winslow cordoned off the area below the apartment with yellow caution tape. Black smoke poured out of two windows on the side of the building, and a firefighter wearing a respirator knocked out a window screen, which fell to the ground below.

Fire Chief David LaFountain, at the scene, said it was unclear what had started the blaze, but it appeared the fire was contained within apartment 7G.

“We don’t know what happened,” LaFountain said, adding that investigators had started examining the scene.

The structure is owned by the Waterville Housing Authority.

Apartment resident Barbara Madore said she was at Walmart and didn’t know about the fire until she returned.

“I just found out when I got back,” Madore said. She was sitting in a wooden gazebo at the back entrance of the apartment building, surrounded by other displaced residents. She had lived in the apartment for two years, she said.

Madore said she hadn’t been cooking food before she left the apartment and had no idea how the fire broke out. She thought her cat was still in the apartment, and no one had told her anything about the fire yet, she said.

John Davis, who lives in a seventh-floor apartment on the other side of the building, said he originally thought that he had set off the alarm by burning a loaf of bread he was defrosting in his oven. He quickly realized that there was a more serious reason for the alarm, he said.

“We could smell it,” he said. “The apartment was already full of smoke.”

Paula Maillet, who also lives in the building, said there were regular fire drills, but never anything serious.

“We’ve had sirens go off once and a while, but it is never anything,” she said.