The men, Egyptian Copts working in Benghazi, were seized by an Islamist militia but handed over to the government. One, named as Ezzat Atallah, 44, died at the weekend after being "tortured", his brother, Effat, told The Daily Telegraph.
The authorities have condemned an attack on the community's priest, but in a sign of their powerlessness in the face of sectarian violence, have taken no further action. After the death of Mr Atallah, four men remain in custody out of 60 detained, while 35 of the rest have been deported for illegal immigration.
"When I visited him he was in a pitiful condition," Ragaa Nagah, wife of one of the four, Emad Seddeek, said. "He was afraid to tell us how he was tortured, but he couldn't see out of one of his eyes.
"They were standing over him and beating him while they asked him to confess and when they were about to give him an electric shock he said, 'Don't do that and I will say anything you want me to say'."
The arrests were revealed in a startling video which sent shock-waves through Egypt's Christian minority. It showed a room full of men, lorded over by Islamist militiamen who have shaved their captives' heads and refer to Christian images sitting on the table in front of them.
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The round-up appears to have been carried out by Ansar al-Sharia, a notorious militia accused of involvement in the death of Chris Stevens, the American ambassador, in the city last September.
That group is not recognised by the authorities, though it operates with impunity, but the captives were then handed over to a government-backed group.
The video makes clear that the incident started as a business dispute.
Muslim stallholders complained that Christians overpaid for their stalls, making it hard for them to compete.
Some of the captors wear the moustacheless beards common to purist Salafi Sunni Muslims.
The seizure of the men followed the arrest of four Christians from different countries for proselytising earlier in February.
Among the recent arrests was a bookseller and evangelist called Shareef Ramses, said to be in possession of "thousands" of bibles. The other four kept in custody, including the dead man, were being investigated because their numbers were on his phone, the Egyptian consul in Benghazi, Ashraf Shiha, said. He denied that the men had been tortured.
"Some police cars stopped by our house," Mrs Nagah, a teacher, said. "They were arresting our neighbour Eissa Ibrahim.
"They asked my husband to come with them to make bail for Eissa before releasing him. But when he went with them they kept him in custody."
The militants then turned their attention to the community's church, attacking the priest-in-charge, Father Pola Isaac, and his assistant.
"They shaved Fr Pola's moustache off, and his head, and beat him up before letting him go," said Yussef Shaker, the son of another of the detainees, Adel Shaker.
The shaving of the moustache suggests the attack was sectarian in motivation.
Previous attacks on the small minority of Christians in Libya, nearly all expatriates, have been put down as one-off incidents. Two Egyptians were killed in December when their church in the town of Dafniya was hit by a grenade.
The incidents have worsened relations between the two countries, already strained by Libyan allegations that Egypt is sheltering Gaddafi regime fugitives and refusing to return stolen money. Copts staged a protest outside the Libyan embassy on Monday night, burning Libyan flags.
The Libyan foreign ministry condemned the attack on Fr Pola as "contrary to the rules of Islam".
Mr Atallah, who died, suffered diabetes and heart problems, and his family said his treatment in prison contributed to his death.
Yussef Shaker said it was true that thousands of bibles had been found, but he added: "Shereef said that Libya was free after Gaddafi and anybody who wants to read anything is free to do so."
