Investigators work the scene of a fire at a three-story apartment complex early Saturday, Dec. 21, 2019 in Las Vegas. The fire was in first-floor unit of the Alpine Motel Apartments and its cause was under investigation, the department said. Photo: AP Photo/David Becker.

LAS VEGAS — A fire in a three-story apartment building in downtown Las Vegas where residents were apparently using their stoves for heat killed six people and forced some residents to jump from upper-floor windows to escape the heavy smoke before dawn Saturday, authorities said.

Investigators reported that the fire started around a first-floor unit’s stove and that residents had told them that there was no heat in the building, which sits a few blocks from downtown Las Vegas’ touristy Fremont Street District.

Residents reported awakening to pounding on doors around 4am.

By the time Matthew Sykes got his clothes on to flee, one end of his second-floor hallway was choked with thick black smoke, as was a stairwell, making it impassable for he and his wife.

“The whole place was like one big black cloud of smoke — couldn’t get down the stairs,” Sykes told The Associated Press.