The eventual death toll of the explosion was thirty-nine men and boys, with a further six severely injured. An inquest jury sitting in the March following the explosion, returned the following verdict; ‘That an explosion of gas occurred at the National Colliery, in the parish of Llanwonno at 6.40p.m. on February 18th 1887, whereby Gareth Griffiths and others lost their lives, and they cannot in consequence of the lack of evidence find out the cause of the explosion or where it started'. The jury did state however that; ‘It is the unanimous opinion of the jury that the management of said colliery was conducted in a loose manner, and not so carefully as the case required'. In his report on the explosion to the secretary of State for the Home Department, F.A. Bosanquet is much more specific as to the causes of the explosion. In this report he pinpoints the exact place and the manner in which he believed the explosion originated. He firstly describes the mine itself as being dry and dusty, ‘common in the Rhondda' mines, with frequent ‘blowers' of fire-damp. The explosion itself was confined to the north side of the mine where none of the men in the workings survived.
He praises the owners of the mine for providing the best safety appliances then available. The mine had two fans for ventilation (one a spare for emergencies), the safety lamps in use were of an ‘expensive description… of admitted excellence', and water cartridges were used for blasting.
However, he is critical of the management of the mine on a number of points, particularly the lack of an efficient means of watering the mine and removing dust which ‘abounded in the mine'. He also criticises the lax monitoring of the use of lamps and explosives in the mine. The actual cause of the explosion he believed was due to a shot fired in the Cwm Nedd district of the mine. He believed the evidence was clear as to this being the origins of the explosion. He reports that on the very day of the explosion, John Jones reported gas in his stall ‘worse than he had ever known it before'. There was such an accumulation of gas that it was sufficient to produce a ‘cap' in a safety lamp some thirty-three yards away at the junction of the stall road with the Risca heading. It was in this area that he believed evidence pointed to a shot having been fired at the time of the explosion. In his report, Mr. Bosanquet believed that the firing of the shot whilst the ‘eight-hour' shift were in the mine, clearly infringed the Coal Mines Regulations Acts of 1872 and he recommended that ‘proceedings should be taken against Mr.Watts, the agent and Mr.Williams the manager, for this breach of the rules'.