Mitchell was hired by Commissioner Bud Selig last March following more than a year of allegations against Barry Bonds, Mark McGwire and other stars. No timetable was set for Mitchell's report.
"When I began, I was, of course, aware that I do not have the power to compel testimony or the production of documents," Mitchell said in a statement Friday. "From the outset I believed that the absence of such power would significantly increase the amount of time necessary to complete the investigation, and it has."
While club officials have testified, Mitchell can't order any of the unionized players to cooperate. No player is known to have testified.
"My investigative staff has conducted hundreds of interviews and received thousands of documents; however, much more work will be necessary," Mitchell said. "Cooperation has been good from many of those from whom we have sought testimony and documents, but has been less than good from some others. This will not affect the result of the investigation, but it has increased the length of time it will take me to complete the investigation."
Mitchell said he won't predict when his probe will conclude.
"We are pressing forward diligently and will conclude our work as soon as possible," he said. "It's important to finish this job as soon as possible, but it's even more important to do it right."
Mitchell said he issued his statement in response to recent inquiries by reporters.