BBC Africa,

Usman Muhammad pauses by the entrance to a tiny Nigerian prison cell and leans his forehead against the wall, unable to speak.

The day before he had been trapped inside the stifling room, measuring roughly 6m (19.7ft) by 3m (9.8ft), with more than 50 other prisoners.

Thirty-three of his cellmates died in the cramped conditions in the prison in Minna, the capital of Nigeria's Niger State - and four died later in hospital.

The mass deaths caused an outcry and sparked protests, including attacks on public buildings, as relatives and locals demanded answers.

Federal officials had initially blamed the deaths on a diphtheria outbreak, but exactly what happened is now the subject of an investigation.

"I have never felt heat like that in my life. When we touched each other's bodies, it felt like fire burning," Muhammad tells the BBC once he can gather his thoughts.

When he was locked inside, there was standing room only amid humidity levels above 90%, which in some circumstances can be dangerous to human health because it prevents the natural cooling mechanism of sweat drying on the skin.

The temperature outside was above 25 °C, and the only ventilation came from a small, high-up window.

"One of us fainted, and we started knocking at the [cell] door. When [a guard] came, we asked him for water. He looked at the guy who fainted and closed the door. That's how they started dying one after the other," Muhammad says.

Like the others who had been detained, the young man, in his 20s, is an artisanal miner, eking out a living searching for precious metals at mine sites.

Officers from the Nigeria Security and Civil Defence Corps (NSCDC) had arrested a large group of miners near Minna last Tuesday and Wednesday.

Drinking urine

The prisoners were held at its detention centre in the city, but the NSCDC has not shared either the names or the number of those it was holding.

As the hours inside the cell wore on, the heat and dehydration became unbearable, and Muhammad says some of the detainees began drinking their own urine and collecting sweat from their bodies to try to stay alive.

He describes seeing people in distress and struggling to breathe.

"People were calling for help, some were already very weak."

But no one came to help, he and others allege.

"While in the cell, we were shouting and telling them that people were dying," another survivor, Adamu Isah, told the Reuters news agency.

"We tried to break down the door when they refused to open it… At one point, a security officer came to the door.

"We were shouting and telling him that people were dying, but he said it wasn't his problem if people died."

Finally, on Thursday, the cell door was opened, and the guards saw the dead bodies.

The NSCDC has not directly responded to the accusations of neglect and maltreatment but has said that 24 of its personnel are now in custody in connection with the deaths, including its suspended Niger State commandant, Suberu Aniviye.

"The people who were killed are not flies. How can this go unnoticed? We will get to the bottom of this," Niger State Governor Umaru Bago told the BBC.

A 10-member committee has now been set up to determine exactly what happened.