The cause of the fire remained unknown, but a witness said he saw construction materials burning before the fire climbed up the scaffolding and quickly spread.

大火原因尚未查明，但是有目击者称，大火先在脚手架上着火，然后迅速蔓延直至大楼建筑材料起火。

youdao

The cause of the fire is still unknown.

飞机失火原因目前仍未所知。

youdao

The cause of the fire was unknown, and it was not clear how many of the injured were children.

火灾原因不明，也不清楚伤者当中有多少是儿童。

youdao

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

VOA: special.2009.08.21

At this time the cause of the fire remains unknown.

BBC: Fire at International Laboratory Services in Shardlow

The cause of the fire is unknown.

BBC: The burning barn