The immediate cause of death is unknown.

造成死亡的直接原因不明。

《牛津词典》

If an unknown person enters a home, the camera will send the owner a warning that will cause an alarm to sound on the owner's smartphone.

如果一个不知名的人进入家中，摄像头就会向主人发送警告，并在主人的智能手机上响起警报。

youdao

Or there may be an as yet unknown cause of some cardiovascular pathology.

也可能有某种至今未知的原因导致心血管疾病。

youdao

The cause is unknown -presumed to be multifactorial- with signs noted in early childhood related to abnormalities in behavior, language, and social development.

FORBES: Stem Cell Therapy To Treat Autism?

The cause of pre-eclampsia is unknown, but it is clear that the placenta fails to invade into the wall of the mother's uterus.

BBC: Treatment for pregnancy danger

She has not aborted the seven-week-old fetus, but it is unknown whether the mistake will cause birth defects or other complications.

FORBES: Medication Mistakes Are A Tough Pill To Swallow