In an interview with the New York Times published on Friday, former New Jersey state governor Thomas Kean said the 2001 strikes on New York and Washington could have been avoided if FBI, immigration and other government agents had done their job properly.
But investigators were still studying whether top members of President George Bush's administration should also share the blame, he added.
"There were people at the borders who let these people in even though they didn't have proper papers to get into this country," Kean said in a criticism of immigration inspectors.
"There were visa people who let these people in," he went on.
"There were FBI people who, when they got reports from Phoenix and Minnesota and elsewhere, didn't think they were important enough to buck up to higher-ups.
"There were security officers at the airports who let these people onto airplanes even though they were carrying materials that weren't allowed on airplanes," Kean said.
Kean's commission of 10 Democrats and 10 Republicans is to give its report in May 2004 on the attacks that left about 3000 dead.