Public Defender Steve Carroll said Tuesday that Oceanside Officer Damon Smith taped interviews for eight years but never turned them in as evidence.
Defense attorneys were unaware of the tapes' existence until they were notified earlier this month by the district attorney's office.
The tapes came to light in April when Smith was testifying in a domestic violence case.
Carroll says Smith's motive is unclear. A police spokesman did not immediately return a call.
District attorney spokesman Paul Levikow says it's too early to comment on the probe.
The discovery was originally reported by the North County Times.
Copyright 2009 The Associated Press.