Abdughafor Ghalandarov, the prosecutor of northern Soghd Province, told RFE/RL's Tajik Service that the attack took place in the Ghayroghum district on 25 January.
Prison warden Bobojan Gadibaiev was killed by the armed assailants, who escaped along with the freed prisoner.
Ghalandarov told RFE/RL that the armed IMU has been increasingly active since an uprising and bloody crackdown in the eastern Uzbek city of Andijon in May. He said the IMU has become more dangerous than the militant Hizb ut-Tahrir, an Islamist group that seeks a global caliphate.
Local and international authorities have labeled IMU a terrorist organization.
Tajik authorities have jailed a number of Uzbeks in recent months on charges of spying and attempting to destabilize Tajikistan, according to the Economist Intelligence Unit.