When firefighters arrived at the predawn blaze at the Alpine Motel Apartments, some residents of the Las Vegas complex had already jumped from the three-story building. People hung out of the windows as black smoke filled the structure and smoke detectors beeped, firefighters told reporters.

“The first thing that you think of is, ‘We’ve got to save those people’ — because a fall of 16 feet or higher can be fatal,” Las Vegas Fire Department spokesman Tim Szymanski said at a news conference.

By the time the flames were subdued, authorities say Saturday’s blaze had become the worst fire fatality incident in the city’s history: Six people were dead and 13 injured.

First responders took 10 people to the hospital, and Szymanski said three people went there by private vehicles. Five of the injured, including a pregnant woman, were in critical condition Saturday afternoon.

Two cats and one dog also died in the fire, and Szymanski said one dog was rescued. Between 30 and 35 people were displaced, and the American Red Cross was helping them to find shelter.

The fire broke out around 4:15 a.m. near a stove in a first-floor unit, Szymanski said. Residents told officials that the 41-unit complex in downtown Las Vegas lacked heat and that people had been using their stoves to warm themselves.

Szymanski said the fire appeared to be accidental, but authorities were investigating whether the building on North Ninth Street had code violations because they did not hear fire alarms going off. He said investigators’ first priority, however, was to figure out what caused the fire.

“I’m sure if any citations are going to be issued — I’m not saying there is, but if there are — it’s probably going to take some time because numerous city departments will be working together and they’ll want to make sure that they’ve covered everything before they issue anything,” Szymanski said.

Although the fire was mostly contained to the unit where it began, Szymanski said the unit’s resident left the door open while evacuating. As a result, Szymanski said, smoke filled the hallways and made visibility “zero,” prompting people to try to exit through windows. Residents’ injuries included smoke inhalation, fractures and broken bones.

Firefighters extinguished the blaze within five minutes of arriving at the scene, the department said on Facebook. Szymanski said the building sustained roughly $475,000 worth of damage. The structure was built in 1972 and had 41 units, Clark County property records show.

The cause of the blaze is under investigation. Las Vegas fire investigators, police homicide detectives and crime-scene investigators, and the county coroner’s office were on the scene, which the fire department said is standard procedure in a fatal fire.

The candles, heating devices and Christmas trees that are common at this time of year significantly increase the risk of structure fires occurring, Szymanski said.

“This is the perfect storm for fires,” he said. “This is the period that makes firefighters the most nervous, in addition to the Fourth of July.”

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