WASHINGTON (AP) — A key senator wants to know whether labor shortages are causing delayed and canceled flights at major U.S. airlines. Maria Cantwell, who chairs the Senate Commerce committee, sent letters Friday to the airline CEOs of six airlines — American, Southwest, Delta, JetBlue, Republic and Allegiant. Cantwell says she is troubled by reports that highlight the role of worker shortages in a surge of delayed and canceled flights. Congress approved $54 billion to keep airline employees on the job, but the airlines have shed thousands of workers through voluntary buyouts and long-term leave. Now some are hiring again to deal with a jump in travel.