It's been announced that Rina Matsuno's death was caused by a heart arrhythmia.
Shiritsu Ebisu Chuugaku's Rina Matsuno was declared "dead on arrival" at the hospital on February 8th. Many netizens speculated on what the cause may have been and it's now been revealed by official sources.
SME Records, the label of the group, cited heart arrhythmia as cause of death. There's various kinds of heart arrhythmia, due to it being a collective name for all disorders related to the rhythm of the heart.
Also read: Shiritsu Ebisu Chugaku's Rina Matsuno Passes Away
This can include the heart beating too much, too little or irregularly. Effectively it's the same, not enough blood gets pumped out to the body and the body suffers lack of oxygen, which can lead to unconsciousness and later on death.
Matsuno was away from her work as an idol on the 7th due to poor health. It was on February 8th, at 5 am the parents of Matsuno noticed something was amiss and called an ambulance. However, Matsuno was already dead by the time the ambulance reached the hospital.