Aljazeera said on Tuesday the unidentified man had died in Autaita jail in the city of Sidi Qasim, where about 1000 prisoners, members of the so-called Salafia Jihadia organisation, staged an open strike.
The strikers deny involvement in the bombings and say they have been tortured.
Abu Usama, a prisoner in Autaita jail, told Aljazeera the prisoners were demanding the Moroccan authorities open an investigation into the violations they had faced in police centres and jails.
"We have used all possible means to prove we are not the real sides, but our efforts have come to no avail," Abu Usama told Aljazeera.
These prisoners say they have faced severe violations and torture. They have been forced to sign police reports and face political trials and long-term harsh sentences, he added, pointing out there was no evidence of charges against them.
"Therefore, we have decided to stage an open hunger strike to the end; we either gain our freedom or martyrdom. We have no other options," he said.
"One of the prisoners has died. The prison has claimed the soul of this man who has been detained unfairly. We hold those who detained us unfairly responsible for his death," he added.
At least 31 people were killed and 12 bombers died in the Casablanca bombings, the country's first such attack in years.
The bombers, while carefully orchestrating five near-simultaneous attacks, appeared to have miscalculated when it came to choosing some of the sites to target.
One attack took place at a Jewish community centre that was closed and empty.
The Spanish restaurant after
the blast in 2003
Another bomber blew himself up near a public fountain in Casablanca's old city, killing three Muslims. He apparently mistook it for a similar fountain at the entrance of a nearby Jewish cemetery.
The deadliest bombing took place at the Spanish restaurant Casa de Espana, where at least 19 people died.
Across town, a security guard and bellboy scuffled with attackers at the Hotel Farah, preventing them from entering the hotel and forcing them to set off the bombs at the entrance. The two employees died in the melee.
In a predominantly residential area, the fifth attack damaged a Jewish-owned Italian eatery and the Belgian consulate.