The details of the principal cause are not known.

主因未详。

《新英汉大辞典》

Drinking is the principal cause of this traffic accident.

酗酒是这起交通事故的主要原因。

youdao

Apart from relapse of a person's disease, GVHD is the principal cause of death in patients who have received donated bone marrow.

除了患者病情复发，移植物抗宿主病是接受骨髓细胞移植病人死亡的主要原因。

youdao

Property rights, she says, could not have been the principal cause because England and many other societies had stable and secure property rights for a long time.

WSJ: Cultural Conversation: The New Theories of Moral Sentiments

Other supporters of the prime minister are calling for Mr Fini, the principal cause of Mr Berlusconi's weakening grip on office, to resign as speaker of the lower house.

ECONOMIST: A lively August for Italy��s politicians

It is true that industry consolidation has contributed to higher fares in some markets, although the principal cause of higher fares over the last several years has been higher oil prices.

FORBES: Three Thumbs Down For American Airlines/US Airways Merger