The White House has insisted that the clemency decision was made on the merits.
In interviews at the time, Governor Bush said he was never influenced by politics when making clemency decisions.
The 1988 governors' report said that of the 56 states and territories studied, 36 gave the governor broad authority to make clemency decisions.
Mr. Clinton's clemency decisions have ignited a set of Congressional hearings and one wide-ranging influence-peddling investigation.
"Recognizing that clemency decisions are a matter of presidential prerogative, the facts of several of these cases raise significant law enforcement concerns," she said through a spokesman.
The letter was among the thousands of documents sent by the White House to Congress in response to subpoenas for information about the President's clemency decision.
But the former president has said all his clemency decisions were made strictly on the merits of the cases.
That is the day when governors traditionally announce clemency decisions.
The clemency decision followed a lengthy failed appeals process over a two-year period.
"These women were entrapped, emotionally and physically," Governor Celeste said when he announced his clemency decisions.