Several software engineers, a baby and a pregnant woman were among those killed in the mishap.
Volvo has reiterated that the fire on board its bus in Mahbubnagar District in Andhra Pradesh on October 30 last year that killed 45 people was not the result of faulty vehicle design.
The Crime Investigation Department in Andhra Pradesh, which investigated the case, this morning stated in its chargesheet that it has found faulty design of the Volvo bus, rash and negligent driving by the driver and the culvert design with iron pipes as the main reasons behind the mishap.
The 400-page report on the probe will be sent to the Union government.
The bus, run by a private operator, was on its way from Bangalore to Hyderabad when it hit a culvert on NH44 and went up in flames. Several software engineers, a baby and a pregnant woman were among the deceased.
The probe findings claim the iron pipe on the culvert pierced into the bus, caused sparks from the battery compartment, igniting the main fuel tank made of plastic.
Ten people have already been arrested in connection with the accident.
Volvo in a statement released this evening has said that its designs meet the relevant regulations and that its bus was driving at a speed of over 100 kms during the accident. It also added that the impact of the accident naturally caused severe structural damage to the bus, and that it did want to state anything further since the matter is in court.
The CID's report points out that the Volvo bus design was faulty, with 300 litre main fuel tank between two front tyres and two supplementary fuel tanks behind the front tyre.
The tanks were made of hard bold plastic and the floor was made of plywood instead of steel, says the report, adding that the seats were made of a highly inflammable material.
The emergency exit doors were blocked to make space for additional seats and the total number of seats was beyond the permissible number, it says.
The bus manufacturers (Volvo India ltd), bus owners (Diwakar Roadlines) and bus operators (Jabbar Travels) were grossly negligent and they endangered the lives of their passengers, the report added.