Mitchell told baseball owners last month that his work has gone more slowly than expected and threatened to seek congressional help if he doesn't get better cooperation in his probe, which started nearly a year ago.
Baseball commissioner Bud Selig said some teams had been more cooperative than others.
"I can confirm that members of my investigative staff have been, and will be, conducting interviews during spring training of various individuals who are involved in baseball," Mitchell said in a statement Wednesday. "Our interviews are being conducted in both Arizona and Florida. "
His staff spoke Tuesday with members of the Minnesota Twins' front office and medical staff, the Minneapolis Star Tribune reported Wednesday.
"The interviews are being conducted at this time because a number of witnesses are together in one place and the spring training sites for the major league clubs are close together," Mitchell said. "It has been and continues to be my practice not to comment on the details of the investigation. We will answer as many questions as possible in our final report."
Mitchell has said he will seek to have his staff interview active players.