Four men who were arrested alongside al-Barghouthi told his family that they were all tied up in painful positions during interrogation, and that intelligence officers demanded to know where the detainees had hidden weapons.
Accused of possessing weapons, Azzam Fehil was arrested on February 10 and held for 13 days in the same detention facility as al-Barghouti.
He told Al Jazeera's Jacky Rowland that he was handcuffed and suspended by his arms for long periods. He also said he was kicked and beaten.
Fehil said he was released on the same day the imam died.
The Palestinian human rights commission made two requests this month to visit the detention centre, but with no success.
"I've seen lately, since June onwards, the problem of ill-treatment and the usage of torture taking place. And we are talking here of all the areas under the Palestinian Authority," Randa Siniora of the Palestinian Independent Commission for Citizens' Rights said.
"With this polarisation and the split - political split - between the West Bank and the Gaza Strip, I think the security agenda has been given priority over many over issues."
The Palestinian Authority has announced an official investigation into al-Barghouthi's death.
"Generally speaking, anyone can make allegations. However, the truth is the marks that you have seen on the wrists of these prisoners are a result of the handcuffs used," General Mossab al-Sa'di, the head of intelligence services in the West Bank, said.
"I imagine that any country in the world that arrests someone who is not a militant or a bomber or even armed; who is merely a criminal, handcuffs them. All of our officers during interrogation or arrest, even if the criminal resists, do not beat them. We have never tortured anyone."
But Hamas officials say they do not trust the government probe.
About 3,000 Hamas loyalists marched at the funeral of the 44-year-old imam on Sunday, accusing the Palestinian Authority of killing him and turning the ceremony into a rare show of defiance against Mahmoud Abbas, the president.
Al-Barghouthi, was among hundreds of Hamas activists to be detained by Abbas's security forces in the West Bank following the violent Hamas takeover of Gaza in June. Dozens remain in custody.
Anger over his death has sparked new clashes between Hamas supporters and the Palestinian police which are predominantly loyal to Fatah.
And with no sign of reconciliation between the two factions, ugly scenes of Palestinian on Palestinian brutality are likely to repeat themselves across the West Bank.