The statement from Mr. McCain’s office said a two-inch blood clot was removed from “above his left eye” during a “minimally invasive craniotomy with an eyebrow incision” at the Mayo Clinic Hospital in Phoenix, “following a routine annual physical.” Surgeons there are not conducting interviews. Mr. McCain’s communications director, Julie Tarallo, said further information would be made public when it became available…

“Usually, a blood clot in this area would be a very concerning issue,” said Dr. Nrupen Baxi, an assistant professor of neurosurgery at the Albert Einstein College of Medicine in New York City…

But many questions have been left unanswered, including whether Mr. McCain had symptoms that prompted doctors to look for the clot. In June, his somewhat confused questioning of James B. Comey, the former F.B.I. director, led to concerns about his mental status, which he later jokingly dismissed by saying he had stayed up too late watching baseball the night before.