At least 17 members of a family were among those killed when a fire engulfed a top-floor apartment in the Gaza Strip during a birthday party, a relative said Friday.

Officials in Hamas-ruled Gaza said Thursday night’s fire in a three-story residential building in the Jabaliya refugee camp appeared to have been started by stored gasoline. They said it was unclear how the gasoline ignited, and an investigation was under way.

Officials put the death toll at 21, making it one of the deadliest incidents in Gaza in recent years outside of violence stemming from the Israeli-Palestinian conflict.

The blaze destroyed the top floor apartment in the building, home to the Abu Raya family.

Mohammed Abu Raya, a spokesman for the family, told The Associated Press that the extended family had gathered for the celebration of twins – the birthday of one of the children and the return of one of the adults from a trip to Egypt.

Abu Raya was speaking at the Indonesian Hospital in northern Gaza, where bodies have been removed and where weeping relatives await the funeral procession to begin.

Abu Raya challenged claims that stored gasoline started the blaze, saying furniture made of flammable materials was more likely to accelerate the blaze.

“The disaster is that no one comes out alive to tell us the truth about things,” he said. “I don’t think it’s stored gasoline.”

Abu Raya said those killed spanned three generations of Abu Raya – a husband and wife, their five sons, two daughters-in-law and eight grandchildren.

The identity of the remaining victims was not immediately known.

Friday evening, the bodies were transferred from the hospital morgue to the mosque before being buried.

Gaza faces a dire energy crisis, largely because of the crippling Israeli-Egyptian border blockade that has existed since Islamist militant Hamas took control of the territory 15 years ago.

People often store cooking gas, diesel and gasoline at home in preparation for the cold season. Previous house fires were caused by candles and gas leaks.