An Egyptian human rights group said on Wednesday that a 19-year-old was tortured to death in custody in the Mediterranean city of Alexandria, while the interior ministry claimed the detainee died of natural causes.

Mostafa Montaser was arrested on 20 July along with three others on drug possession charges, according to the interior ministry on Tuesday.

The three were interrogated by police and the prosecution ordered their detention pending further investigations.

“The man in question, known for doing drugs, suddenly felt unwell and was transferred to [a hospital] where he died naturally,” the interior ministry statement added, without referring to the cause of death.

The demise of the detainee raised questions of possible foul play by Egyptian police, who have been routinely accused of using torture and beatings against prisoners.

“The Egyptian Network for Human Rights [ENHR] monitored the detention of Mostafa Montaser and his fellows earlier on 18 July until he was buried 10 days later with stains of blood on his body, unlike the interior ministry’s claim that he died of natural causes,” the statement read.

An autopsy report has not been released but the rights group called on the prosecutor general to open an investigation into Montaser’s death.

Egypt routinely tortures detainees – sometimes fatally – and keeps them in horrifying conditions, human rights groups say, with many prisoners held without trial for years.

In 2010, Khaled Said was tortured to death by two policemen in Alexandria. The disturbing incident was seen as a symbol of police brutality and one of the main drivers behind the 25 January Revolution which toppled long-time autocrat Hosni Mubarak.

The regime of President Abdel-Fattah Al-Sisi has been overseen the worst human rights violations in decades, according to local and international human rights groups.