WARSAW, Poland (AP) — A fast-moving fire tore through a three-story building housing homeless families in northwestern Poland early Monday, killing 21 people and injuring 20 more, including some who leaped from windows to escape the flames, officials said.

Firefighters finished picking through the charred hull of the shelter, which was run by the local government, in search of more bodies and clues to what caused the fire. Poland's president, meanwhile, visited the site and suggested the construction appeared sub-par and may have contributed to the intensity of the blaze.

The fire broke out around 1 a.m. in the shelter for people waiting for government housing in Kamien Pomorski, 370 miles (600 kilometers) northwest of the Polish capital and near the Baltic Sea coast, said Pawel Fratczak, a spokesman for Poland's national firefighters.

Fratczak said 21 people died in the fire, but noted that "it is going to be hard to identify many of them quickly because they are so badly burned."

Footage broadcast on TVN24 television taken when firefighters first arrived on the scene showed the stone and metal building engulfed in flames, while in later video firefighters picked through the building's charred, gutted shell with axes.

"I couldn't breathe, so I opened the window and looked out, and the door was on fire because it was essentially made of paper," a survivor told reporters at the scene in footage shown on TVN24.

"I put my pants on and boots, and a friend gave me a jacket, and there wasn't any more time because it was burning like a torch. Some people jumped into the trees," the unidentified middle-aged man said.

It was not immediately clear what caused the blaze, but police and fire investigators were examining the scene for clues. It was also unclear whether the building had fire alarms or a sprinkler system.

However, the speed at which the building went up in flames led many people, including President Lech Kaczynski, to question the shelter's design and materials.

"If the building burns at that fast a rate, and if nobody poured gasoline over the entire building and set it alight on purpose — and that theory doesn't exist right now — then there must have been some fundamental flaw in the construction," Kaczynski told reporters.

The president also declared three days of national mourning to start at midnight in memory of those who died.

There were 77 people registered as residents in the building at the time of the blaze. Fratczak said not all were yet accounted for, and that the death toll could rise.

The injured, many of whom suffered broken bones after jumping out of windows on the upper floors to escape the blaze, were taken to local hospitals. Two people were being treated for burns, including an 8-month-old baby.

The injuries are not considered life-threatening, Fratczak said.

"I'm in shock. The most important thing is that we're alive and that the kids are alive," a woman lying in a hospital bed with burns visible on her face told TVN24 through tears.

"I'm just worried what happens now. We lost everything. We have nothing. I hope there is somebody who will help not only us, but everyone who has suffered from this because this is truly a tragedy."

Prime Minister Donald Tusk flew to Kamien Pomorski early Monday to inspect the damage, and promised aid for those who escaped the blaze. Many of the residents had been living at the temporary housing shelter for a long time as they waited for a permanent apartment in Poland's crowded social housing network.

"The help will be full and will ensure the housing needs of those who survived this disaster," Tusk told reporters in front of the still smoking building. "The most important thing is to help those who survived."