HRW calls on Egypt to reveal whereabouts of missing Islamist
Hossam Menoufy Sallam was travelling on January 12 on a direct flight from Khartoum to Istanbul when his flight made an unscheduled landing in Luxor in Egypt's south, HRW said.
The unplanned landing took place as a "routine procedure" after the smoke detection system went off in the cargo cabin, the Khartoum-based Badr Airlines said in a statement.
"After all passengers disembarked to the transit lounge, security officers summoned Menoufy and checked his passport and travel documents," HRW said in its report.
"He was last seen by witnesses in the custody of Egyptian officials," the New York-based group added.
Egypt's interior ministry issued a statement on January 15 saying that Menoufy was held in custody pending investigations.
"The Egyptian government should immediately disclose Hossam Menoufy's whereabouts and allow his lawyer and family to see him," said Joe Stork, HRW's deputy Middle East director.
"Forcibly disappearing him is a serious crime," he continued.
Local media in Egypt reported that Menoufy was a leading figure of the militant group Hasm, believed to be a breakaway faction of the outlawed Muslim Brotherhood.
Egyptian activists suggested that the forced landing of the Sudanese flight points to the close cooperation between Egypt and Sudan, where the military led by General Abdel Fattah al-Burhan staged a coup on October 25.
Egypt launched a harsh crackdown on the Muslim Brotherhood group after the 2013 military ouster of late president Mohamed Morsi, who hailed from the group.
Thousands of the group's supporters have since been jailed on terror-related charges.
The Brotherhood was designated a "terrorist organisation" in late 2013. The group has consistently denied any links with violence.
On Sunday, an Egyptian court sentenced to death 10 Brotherhood members found guilty of violence against security officers in 2015, a judicial source said.
The case was referred to the Grand Mufti, Egypt's top theological authority - a formality in death penalty cases - before the court meets on June 19 to confirm the sentences.