Morsi's youngest son dies in Egypt of heart attack
Abdullah Morsi, who was in his mid-twenties, had emerged as a spokesman for the family after his father died in an Egyptian jail on 17 June.
The younger Morsi, who died on Wednesday, had accused security officials of being behind his father's death.
Days after his father died, Abdullah identified several figures, including current Interior Minister Mahmoud Tawfiq, his predecessor Majdi Abdel Ghaffar as well as Mohamed Shereen Fahmy, the judge who oversaw the ex-president's trial, as "accomplices" in the "assassination of the martyr, President Morsi".
Morsi, a member of the Muslim Brotherhood, was elected in Egypt's first-ever presidential election in 2012, after a popular uprising overthrew longtime leader Hosni Mubarak.
A year into his rule, Morsi was overthrown by the Egyptian military in a coup led by his own defence minister, Abdel Fattah el-Sisi, who would go on to become president.
Since then, Sisi has embarked on a massive campaign to crack down on dissent and free speech. He has also banned Morsi's Muslim Brotherhood, blacklisting it as a terrorist group.
Morsi remained in jail after his overthrow until his death during a court session in June.
A 2018 report by members of the British Parliament had warned that the "cruel, inhuman and degrading" conditions of Morsi's detention, including lack of medical care, may lead to his death.
In his final Twitter posts, Abdullah mourned his father and shared his speeches, calling for justice for the late president.
The younger Morsi was detained briefly late last year over charges of spreading false news. He also spent a year in jail after the coup in 2015.
Mohamed Soltan, a former political prisoner in Egypt, mourned Abdullah in a Twitter post recalling the late activist's courage when the two crossed paths in prison in 2015.
"He was in the cell next to mine in Leeman Tora prison for eight months from the day of his arrival until my departure," wrote Soltan, who has started an organisation in Washington to advocate for political prisoners.
"He used to come to my cell to keep me company and encourage me. He would rush to my cell to help me from my bed to the wheel chair on visitation days. A brave friend worth 1,000 men despite his age."
Abdullah's older brother, Osama, who faces charges of planning to organise protests, has been in jail since his arrest in 2016.
Human rights groups estimate that Sisi's government has jailed as many as 60,000 dissidents since a 2013 coup against Morsi.