Sattar Beheshti was arrested because of his criticisms toward the corrupt and non democratic government of Iran. He was arrested on 10/30 and he never left custody alive.
The iranian Parliament has ordered an investigation of that accident. Too little, too late. And one may be suspicious of what that "investigation" will come out in a country where arbitrary arrests, vanishing prisoners and violations of basic human rights are every day's facts.
Here is the full article on Reuters.