The supplier, Delphi, did not permit its employees to be interviewed by Anton R. Valukas, the former United States attorney who conducted an internal inquiry into G.M.’s handling of the switch problem and this month produced a scathing report on the automaker’s corporate culture.

But the Senate Commerce Committee, which has the power to subpoena witnesses, unlike Valukas’s company-financed investigation, is expected to make Delphi a subject of a hearing soon. The hearing has not yet been scheduled, though.

Delphi employees have had informal meetings with congressional staff members, according to a person briefed on the matter.

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