Five members of the Saudi religious police are said to be under investigation after a man in their custody died on Friday in unclear circumstances. Ahmed Al-Bliwi, a 50-year old former Saudi border patrol guard, was stopped near an amusement park in the northwestern city of Tabuk. A retiree, he used his car as a private taxi in order to ferry passengers about and earn some extra income. The religious police, known in Arabic as the Muttawa, detained Bliwi "for letting a woman into his car," a police source told the Saudi English-language daily Arab News. Saudi Arabia frowns upon fraternization between men and women who are not relatives. The police took Bliwi to a detention center, where he was interrogated. Several hours later, he was rushed to a local hospital and pronounced dead on arrival. While Saudi authorities claim that Bliwi died of a "sudden heart attack" while in custody, his relatives believe he was beaten and tortured by the police, the stress of which brought on cardiac arrest. They are demanding that an autopsy be conducted in order to determine the precise cause of death. Bliwi was the second Saudi citizen to die while in Muttawa custody. Last month, police in Riyadh opened an investigation after a man being held on charges of "promoting alcohol" was allegedly beaten to death by Muttawa officers. Officially known as the Commission for the Promotion of Virtue and Prevention of Vice, the Muttawa are estimated to number some 5,000 officers. An arm of the Saudi government, they are responsible for ensuring the strict enforcement of Islamic dictates in the desert kingdom. Saudi press reports have suggested that the religious police are increasingly coming under fire for their activity, as a growing number of Saudis chafe under their restrictive practices.