Keighley's Wales international Danny Jones died of a cardiac arrest triggered by hereditary heart disease, it has been confirmed.
Jones collapsed during Keighley's League 1 game at London Skolars last Sunday and efforts to resuscitate him failed.
His family received the initial results of the post-mortem into the death on Thursday when it was revealed that the heart disease was not revealed when the player underwent an ECG scan in late 2014.
Keighley chief executive Gary Fawcett said: "Whilst the findings of the post-mortem do provide an explanation for why he died, they allow for little comfort for Danny's family or all those who knew him.
"His tragically premature death is still very painful for everyone, not least because his heart condition could not even be detected by the ECG.
"All we can do now is mourn his passing, celebrate his life and do everything we can for Danny's wife Lizzie, their children Bobby and Phoebe and their wider family."
Jones leaves a wife Lizzie and five-month-old twins.
A fund set up by the Rugby Football League's benevolent fund has so far raised more than £77,000 through 4,700 donations and will be boosted further by proceeds from Keighley's next match, against Coventry at Cougar Park on Sunday.
Organisers had set a target of £100,000 but Fawcett wants it to reach £1million with the support from football and rugby union.
"I know that there is a huge amount of fundraising to be done over the coming weeks," Fawcett said.
"We all know that there will be a wave of donations from all the rugby league clubs, in particular, over the course of next week which will smash the RFL's current £100k target for the fund. This is fantastic and amazing and yet it is simply not good enough.
"I met Lizzie (on Wednesday). She said that she didn't want Danny's death to be in vain and would like to see the changes in sport that would ensure that what happened to Danny on Sunday would never happen again.
"The fund will provide future financial stability for Lizzie and the twins and it will give Lizzie the financial capability to enable the necessary changes in the sport to ensure that what happened to Danny will not happen again.
"The fund needs to be much, much bigger for the objectives to be achieved. Our club is planning at least five months fundraising and, although we will raise a significant amount of cash, it still will not be enough.
"(On Wednesday night), during my interview on Boots 'n' All, I stated an objective of £250k for the fund. Overnight I have thought about it and the fund needs to be over £1m."
Coaches and players throughout rugby league have joined a growing call for cardiac screening, which is currently mandatory only in Super League, to be rolled out across the game.
Rugby league players, fans and administrators came together on Monday to raise in excess of £10,000 to support the family of Danny Jones - the Keighley player who died on Sunday of a suspected cardiac arrest.