Life insurance claim rejected - suspicious circumstances

My brother passed away 6 months ago due to sudden cardiac arrest. He had a term insurance policy of 50 lakhs crores from HDFC Life which was bought 4 years ago. When we filed the death claim, the insurance company rejected it saying suspicious circumstances. We have submitted all documents including death certificate, policy papers, hospital records, and FIR. The insurance company is not giving a proper explanation. We are a middle-class family and this claim amount is very important for us. What can we do now? Can we file a complaint with IRDAI? Should we hire a lawyer? Has anyone successfully challenged a claim rejection? Please guide us.