al-hijji family left cars and food behind when they fled in august 2001 .
fbi agent 's report said family of saudi advisor had ` many connections ' to hijackers , some of whom were said to have visited the house .
agency now says report ` poorly written ' and ` unsubstantiated '
9/11 review commission accepted fbi verdict , did not interview agent .
congress document redacted by bush administration pointed ` very strong finger ' at financing from saudi arabia .