WASHINGTON — The failures of Bridgestone/Firestone tires linked to 148 U.S. traffic deaths were caused by a combination of manufacturing and design flaws in the tires that can be exacerbated by overloaded vehicles, a professor hired by the company said Friday.

        Sanjay Govindjee, a civil engineer at the University of California-Berkeley, reached conclusions similar to those arrived at in December by the tire company's internal investigators.

        Mr. Govindjee did not answer some of the most contentious issues in the case, such as whether the recall initiated after the problem became public is broad enough to cover all the defective tires and to what degree the Ford Explorer was at fault.

        Most of the accidents were Explorer rollovers that occurred after a tire failure. Bridgestone/Firestone contends that the inflation level recommended by Ford was too low for a vehicle the size and weight of the Explorer.

        “I didn't really do an investigation of whether Ford or Firestone are to be blamed here,” Mr. Govindjee said in a conference call with reporters. He said that would be determined by a federal investigation under way at the National Highway Traffic Safety Administration.

        Ford contends that the Explorer is not at fault, and company spokesman Ken Zino said Mr. Govindjee's report uncovered nothing new.

        Bridgestone/Firestone recalled 6.5 million ATX, ATX II and Wilderness AT tires on Aug. 9. Mr. Govindjee would not assess the scope of the recall, though many advocates and NHTSA have pushed for an expansion.

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