Former US President Bill Clinton testified behind closed doors to the national commission investigating the September 11, 2001, attacks on Thursday, the panel said.
"The commission found the former president forthcoming and responsive to its questions. We appreciate the excellent co-operation he and his associates have given to us," the commission said in a brief statement.
Commission chairman Thomas Kean, a former Republican governor of New Jersey, and vice chairman Lee Hamilton, a former Democratic congressman from Indiana described Clinton's responses during a four-hour meeting as "frank" and "constructive."
In a joint appearance on the PBS program "NewsHour with Jim Lehrer," Hamilton said Clinton made "a lot of very constructive suggestions to us as to how to put the report together and what kinds of recommendations to make."
But Kean said it was unlikely that what Clinton told them would be made public. "A lot of it is classified, and it was discussing classified materials. So we took very careful notes and they'll be used to inform the report, but there probably will not be a release of the material," he said.
Clinton appeared before the panel in closed session shortly after White House national security adviser Condoleezza Rice testified at an open hearing.
A spokesman for the former president said Clinton was pleased to have the opportunity to address the panel.