Last year, Iran said it detained several Iranian nationals whom it accused of transferring nuclear secrets to foreigners, namely the United States and Israel. The suspects have gone to trial, but Iran has kept the verdicts a secret.
Soltani is a lawyer representing the family of an Iranian-Canadian photojournalist Zahra Kazemi, who died in mid-2003 of injuries suffered in Iranian custody.
Soltani's attorney, Mohammad Dadkhah, said he suspects the true reason for Soltani's arrest was his work in investigating the case of Kazemi, who died in prison under suspicious circumstances.
An Iranian court in mid-2004 moved to end the trial of the key suspect in her death, prompting an outcry from Canada and Kazemi's legal team, which is led by Nobel Peace laureate Shirin Ebadi.
Also on Iran:
"EU Warns Iran Over 'Damaging' Nuclear Step"