Haunting accounts of torture in newly found detention centre lead to calls for an investigation into what experts say could be among the worst atrocities of Sudan’s civil war

• Evidence of torture found as detention centre and mass grave discovered outside Khartoum

Lying between the makeshift graves is a mattress, a large bloodstain visible in the midday sun. A name is scrawled in Arabic on its ragged fabric: Mohammed Adam.

Who was Adam? Had he ended up here, in a bleak corner of a remote military installation in Sudan’s Khartoum state? Had his body been stretchered on the mattress from the detention centre nearby and dumped into one of hundreds of unmarked graves?

Exclusive: What appears to be a vast burial site found at former Rapid Support Forces base in Sudan, while rescued detainees speak of torture, starvation and deaths of fellow inmates

• ‘Here you will die’: detainees speak of executions, starvation and beatings at hands of Sudan’s Rapid Support Forces

More than 500 people may have been tortured or starved to death and then buried in a secret mass grave north of Khartoum, according to evidence seen by the Guardian.

A visit to a base belonging to the paramilitary Rapid Support Forces (RSF) shortly after it was retaken by the Sudanese military found a previously unknown detention centre, with manacles hanging from doors, apparent punishment chambers and bloodstains on the floor. Accounts from people held at the detention centre describe being repeatedly tortured by their captors.