The seaplane that crashed off Miami Beach last month, killing all 20 people aboard, had fatigue cracks in both wings, a preliminary federal report said Monday.

The right wing of the Chalk's Ocean Airways plane separated from the fuselage shortly before the Dec. 19 crash, and investigators had earlier found cracks on the right wing's support beam. But the new report by the National Transportation Safety Board on Monday revealed that the left wing had fatigue cracks as well.

The 58-year-old, G-73 Turbine Mallard plummeted into the ocean minutes after taking off for the Bahamas.

The NTSB's final report will be completed later this year, NTSB spokesman Paul Schlamm said.

After the crash, the Federal Aviation Administration grounded all G-73 seaplanes until they could be inspected. Chalk's was the only commercial operator of the planes.

The aircraft was built in 1947 as a Grumman Mallard. In 1979, it was modified to increase seating capacity from 10 to 17 passengers, and to replace the original engines. Chalk's began using the planes the following year, investigators said.

A message left after business hours Monday for Chalk's general manager, Roger Nair, was not immediately returned.