WASHINGTON?Thirteen of the hijackers in the Sept. 11 attacks entered the United States legally, but no entry records exist for the other six, raising doubts about who they are as well as how they got in, the nation?s immigration chief said Thursday.

?Six of the individuals, we can find no record of them period. That?s not just INS, that?s everywhere,? said James Ziglar, head of the Immigration and Naturalization Service, as his agency released records.

FBI Director Robert Mueller said Sept. 27 the bureau has been working to find the hijackers? true identities. He said some of the names attached to the photos of the 19 released by the FBI may not be correct.

?Our investigation has reached out to a number of countries? in an effort to determine the identities, Mueller said.

INS spokesman Bill Strassberger said that regarding some of the hijackers, ?You’ve got the possibility of stolen identity, multiple identities or someone who sneaked across the border.?