The cause of the fire is not yet known.

火灾的原因尚不清楚。

《牛津词典》

Fire investigators have responded to the scene but have not yet determined the cause of the fire.

火灾调查人员已经对现场情况做出了回应，但还没有确定火灾的原因。

youdao

The Paris prosecutor's office has opened an inquiry into "involuntary destruction by fire" indicating they believe the cause of the blaze was accidental rather than criminal.

巴黎检察官办公室对“火灾造成的非自愿破坏”展开了一项调查，这表明他们认为造成火灾的原因是意外事故，而不是犯罪行为。

youdao

Cause she's been through the fire before And I believe she can take a whole lot more.

VOA: special.2010.07.02

Then on April thirtieth, two thousand seven,a huge fire of unknown cause almost completely destroyed South Hall.

VOA: special.2009.08.21

Any new heat or fire could cause the leaking well and the surrounding area to explode.

VOA: special.2009.03.22

And Syria's failure to maintain a cease-fire could cause enough frustration to shift the political landscape.

CNN: Can U.N. monitors help forge peace in Syria?

If you asked my wife, she would say "yes" because for awhile, during that minute or so after the pop, she thought that the explosion and fire might cause a hole in the side of the plane that would decompress it.

CNN: A 'lucky to be alive' passenger's story

The Scottish Fire and rescue Service said the cause of fire was not yet known.

BBC: Firefighters tackle blaze at Stirling Superglass factory