AMR Corp's American Airlines operated jets later found to have substandard repairs, and federal regulators are probing allegations that at least one plane was considered unsafe to fly at normal cruise altitude, the Wall Street Journal reported on Saturday.

The newspaper, citing people familiar with the matter, said the latest moves by the Federal Aviation Administration indicate the agency is expanding its probe into suspected structural problems with rear bulkheads on some of American's fleet of MD-80 jets, the newspaper said.