It’s clear that from the statements from the Dutch anti-terrorist coordinator Erik Akerboom as well as the Yemeni authorities is that highly likely that Abdulmutallab had 2 passports:
1. Nigerian passport with valid multi-entry USA visa as well as expired Yemeni visa
2. Italian passport that was used to travel from Yemen to Nigeria and potentially from Nigeria to Amsterdam
It seems unclear as to what passport (if any) he used in attempting to enter the U.S.
The options are:
1. Nigerian passport with the multi-entry USA visa
2. Italian passport that would not require a visa to enter the USA.
3. No passport at all courtesy of the well-dressing Indian handler in the Amsterdam airport. This would also indicate his flight to the USA would have been booked under a different name from his Nigerian passport and different from his Italian passport.