Pratt spokesman Matthew Bates declined comment on any specific results or conclusions, but said the company was making good progress in its investigation of what caused the crack.

“We have made significant progress ... and believe we’re very close to determining root cause,” Bates said.

One defense official said it was premature to speculate about the cause of the crack until the full battery of structural tests had been completed.

The official, who was not authorized to speak publicly, said Pratt was expected to deliver a comprehensive analysis of the test results to Pentagon officials no later than Thursday evening.

Two sources familiar with the investigation said the fan blade tests would include a “destructive” test that would cut into the turbine blade to better understand how the crack developed.

Engineers believe the crack is either a “creep rupture along a grain boundary” that was caused by prolonged exposure to high heat, or that it was caused by an anomaly during the metal casting process, the sources said.

The F-35 program, initially meant to start operating in 2012, is overdue and well over its original budget, but defense officials say it is making progress. They argue that the current grounding -- and a separate issue involving the plane’s temperature control unit -- are normal occurrences during the development phase of a any new warplane.

The delays are causing problems for countries like Australia, which was due to buy 100 of the radar-evading F-35s, but is now considering whether to buy 24 more Boeing F/A-18 Super Hornets instead.