2 sentences

1. 警方正在调查这起事故的起因。

   jǐng​fāng zhèng​zài diào​chá zhè qǐ shì​gù de qǐ​yīn.

   The police are investigating the cause of the accident.

   Source: Dong Chinese Dictionary

2. 关于这起事故的起因，有关部门正在深究。

   guān​yú zhè qǐ shì​gù de qǐ​yīn, yǒu​guān bù​mén zhèng​zài shēn jiū.

   Relevant departments are conducting an in-depth investigation into the cause of this accident.

   Source: Dong Chinese Dictionary