The FBI is reported to believe up to six planes were intended to take part in terrorist attacks on New York and Washington.
The bureau is considering whether a fifth suicide hijacking was foiled by the cancellation of a flight from Boston to Los Angeles.
The Guardian says there is also growing evidence a sixth attack was planned.
A cell of terrorists was intending to hijack a flight due to leave San Antonio in Texas last weekend.
American Airlines flight 43 from Boston was cancelled at the last minute because of technical problems. The names of passengers are now being checked.
The paper says investigators in San Antonio have arrested a Saudi-born doctor who worked at the University of Texas health science centre.
He reportedly did not turn up for work last Tuesday and is on a 180-name FBI watchlist of people suspected of having links with the 19 hijackers. He is suspected of providing technical or financial assistance to the suicide groups.
The doctor's name also appeared on a passenger list for a United Airlines flight due to leave San Antonio for San Diego via Denver on Saturday.
The Guardian says investigators believe he may have been due to meet two men but they were arrested in Fort Worth, Texas, last Wednesday on a train bound for San Antonio.
Two of the hijackers who died last week in the crash in Pennsylvania have been traced to addresses in San Antonio.