Planned Parenthood told U.S. congressional leaders on Thursday that manipulations and deletions used in the editing process of secretly recorded videos slamming the organization rendered the tapes unreliable for government inquiries.

In an 11-page letter to Republican and Democratic congressional leaders, the reproductive healthcare group's president, Cecile Richards, detailed the findings of an analysis conducted by research firm Fusion GPS and commissioned by Planned Parenthood.

In recent weeks, the Center for Medical Progress has released eight videos showing Planned Parenthood technicians gathering fetal tissue from abortions.

The anti-abortion group has said the videos showed Planned Parenthood officials discussing the illegal sale of aborted fetal tissue. Planned Parenthood has said it has done nothing wrong.

Richards' letter comes as four congressional committees are conducting investigations into her organization. When Congress returns from summer recess on Sept. 8, efforts to halt federal funds for the organization could be part of budget negotiations.

The letter said there were multiple instances of what it called deliberately deceptive edits, inaccurate transcripts and missing footage.

In the accompanying 10-page report by Fusion GPS, video forensic experts tallied 42 instances of unrelated questions and answers being spliced together in the first five videos, creating the appearance of seamless conversations. The edits "substantively and significantly" altered the meaning of the dialogue, the report said.

It was not immediately known how much Planned Parenthood paid Fusion GPS for its research.

Analysts found transcripts had also been altered, at times changing questions by actors "to make it seem less like they are baiting Planned Parenthood staff into making unethical statements," the report said.

In what the Center for Medical Progress called full-length footage, analysts found large portions had been removed, including approximately 30 minutes' worth of video filmed in each of two clinics, the report said.

"Any time someone has made undisclosed changes to an audio or video file, that renders the file unreliable," lead investigator and former Wall Street Journal investigative reporter Glenn Simpson said during a media conference call.

In response, the Center for Medical Progress issued a statement calling the new analysis a "desperate, 11th-hour attempt" to discredit the videos, saying the edits were made to remove bathroom breaks and downtime between meetings.

The Center for Medical Progress has indicated that it will release more videos in coming weeks.