Amir Khilaif al-Inizi died of a heart attack at the military hospital late on Tuesday evening, according to Kuwaiti news agencies. He was aged 29.
Al-Qabas news told Aljazeera.net that al-Inizi "had apparently complained of having problems breathing on Tuesday afternoon after which he was taken to the military hospital".
Lieutenant-Colonel Adil al-Hashash, the head of public relations at the Interior Ministry, added that Amir died at a military hospital "due to a collapse in [his] blood circulation".
It has also been reported by state news agencies that he complained of breathing problems while being interrogated earlier on Tuesday.
A Kuwaiti journalist speaking to Aljazeera.net said al-Inizi was arrested on 31 January and was suspected of being a ring leader of a group known as The Lions of the Island.
The group is said to have planned attacks on US convoys heading for Iraq, and is blamed for a number of shootouts in the country. They are also said to be critical of the governments' support of the US in the region.
Al-Inizi's group was said to be
plotting attacks on US convoys
Another alleged ring leader, his younger brother and purported right-hand man, Nasir al-Inizi, was among several others killed in what Kuwaiti security forces said was a security operation.
Amir al-Inizi was arrested with seven others after a nine-hour shootout which left five people dead, including a policeman. Seven people were arrested thereafter.
Until a few months ago he was the imam of al-Jahra Mosque, 40km northwest of Kuwait City, when he was reportedly dismissed by the Ministry of Islamic Affairs because of what the ministry deemed as "his extreme views".
His wife and 15 others have also been detained and are still being interrogated by Kuwaiti security officials.
Security forces are on high alert after a number of attacks throughout the country.
Cause of death?
Speaking to Aljazeera from Kuwait, Fahd al-Khana, member of the Committee for Human Rights in Kuwait's parliament, said: "We heard that Amir Khilaif al-Inizi died in police custody."
He questioned the cause of death and said the interior minister would be held responsible for al-Inizi's death if it was found that he died because of torture or ill-treatment at the hands of security forces.
"Kuwait's security apparatus is well-aware that torture is illegal and that the interior minister has in numerous meetings pledged that suspects would not be tortured"
Dr Fahd al-Khana,
Member of Committee for Human Rights in Kuwait's parliament
However, "if the death was natural, then it is God's will", he said.
"We are still waiting for independent confirmation regarding the cause of death."
'Torture is illegal'
Al-Khana further said, "Kuwait's security apparatus is well-aware that torture is illegal and that the Interior Minister has in numerous meetings pledged that suspects would not be tortured.
"We hope that his assurance will be upheld and that the deceased suspect was not tortured in any way. Even though we denounce terrorism, the principles of justice have to be maintained.
"The rights of people should be preserved in accordance with the law," he said.
The lawyer who defends detainees and prisoners who say they were tortured by the authorities, Usama Munawwir, has also been arrested in Kuwait.
A number of Kuwait securitymen
have died in the recent clashes
When asked if the lawyer's arrest was a violation of human rights, al-Khana said: "Munawwir was arrested and the attorney-general has extended his detention indefinitely.
"Since the judiciary is dealing with his case, we cannot interfere. We are certain he will be given a fair trial and will be free to defend his clients without intimidation".
He also said: "We have confidence in the judicial system and the security aparatus."