The US Bureau of Prisons discovered the tapes in February, 14 months after the inspector-general found that staff members had abused some detainees at the Metropolitan Detention Centre.
Some tapes from the centre contained conversations between lawyers and their clients, inspector-general Glenn Fine said on Monday in a report to Congress.
Fine is looking into the detention centre's failure to produce the tapes during his investigation.
Lawyers for the Legal Aid Society are suing detention centre officers for secretly videotaping their conversations.
The lawyers say they were assured by the prison that the attorneys' conversations with their clients were not being taped, even though video cameras were on the walls.
Evidence from the hundreds of tapes was incorporated into the Bureau of Prisons' disciplinary review of the staff's treatment of detainees.
Among complaints the inspector-general is looking into at federal prisons regarding people taken into custody after the 11 September 2001 attacks, is a case where while an inmate was at prayer, an assistant warden allegedly entered his cell and ordered a corrections officer to confiscate his prayer rug and Quran and to dispose of the items in a rubbish incinerator.
A Bureau of Prisons inmate alleged that a corrections officer ordered him to drop his Quran on the floor outside his cell. The officer is said to have kicked the Quran and walked away.