Four flights lost; 266 on board

(CNN) -- American and United airlines each lost two flights in Tuesday's air crashes, the carriers said Tuesday. The four flights carried a total of 266 people -- 233 passengers, 25 crew members and eight pilots.

The FBI, which has personnel at airports and crash sites investigating who is responsible for the crashes, is operating on the assumption that each flight was hijacked, federal officials said.

No credible group has claimed responsibility for any of the events Tuesday, officials said.

American Airlines reported two jetliners lost in "tragic incidents":

· Flight 11, flying from Boston, Massachusetts, to Los Angeles, California, had 81 passengers, nine crew members and two pilots aboard. The Boeing 767 is believed to have been one of the two planes that crashed into the World Trade Center in New York City.

· Flight 77, flying from Dulles Airport outside Washington, D.C., to Los Angeles, had 58 passengers, four crew members and two pilots. It was a Boeing 757, and crashed into the Pentagon.

The other two planes were United Airlines airliners:

· Flight 93, flying from Newark, New Jersey, to San Francisco, California, crashed near Shanksville, Pennsylvania, about 80 miles southeast of Pittsburgh, Pennsylvania. The plane, also a Boeing 757, held 38 passengers, five crew members and two pilots. Initial reports indicated no survivors.

· Flight 175, from Boston to Los Angeles, contained 56 passengers, seven crew members and two pilots. This plane crashed into the second World Trade Center tower.

The Boeing 767 twinjet is the most widely used plane across the Atlantic, according to Boeing. Depending on its configuration, a 767 can seat up to 375 passengers in its largest version, the 767-400ER, which went into service last year. Maximum cruising speed of the 767s is Mach 0.80, or about 530 mph at its cruising altitude of 35,000 feet. The first 767 went into service on September 8, 1982.

The Boeing 757, depending on configuration, can seat up to 239 passengers (757-200) or 289 passengers (757-300). Maximum cruising speed is Mach 0.80, or about 530 mph at a cruising altitude of 35,000 feet. The first 767-300 was put into service in March 1999.