INDIANAPOLIS -- An investigation is under way after an early-morning apartment fire sent about 20 residents into the streets early Thursday morning.

The fire broke out at about 1 a.m. at St. Regis apartments at 14th and Pennsylvania Street.

When Indianapolis firefighters arrived at the scene, they found flames shooting from a second-floor apartment.

The person in that apartment got out safely, and no one was seriously injured.

Residents said it was difficult to find a safe way out of the six-floor apartments because of thick smoke.

Fire investigators said smoke alarms sounded in individual apartment units, but they weren't sounding everywhere in the building.

"The fire prevention people will be here later today to test out the fire alarm system and make sure everything is fully operational," said Indianapolis fire Lt. Mike Estridge. "It appears that … there are pull stations in the hallway, and none of those had been actually activated."

"I was just sitting in the dining room, and the fire department came. No alarm came on," said resident Debra Cantrell. "I didn't know what was going on. They came and got me and brought me down."

A 62-year-old man lived in the apartment that caught fire. He told investigators that he stepped out of his apartment, then returned to find the area around a sofa burning.