Jim Tressel resigned as Ohio State University's football coach  on Monday, ending weeks of steadily mounting pressure on both him and the university in the wake of revelations that Tressel failed to act despite knowing that players had violated National Collegiate Athletic Association rules. Ohio State announced in March that it had suspended and fined Tressel  for failing to tell administrators or the NCAA that players had sold team memorabilia and received free tattoos worth thousands of dollars. Although Tressel's two-game suspension grew to five to equal the penalty the NCAA imposed on players, Ohio State had come under increasing pressure to dismiss the coach for his role in the embarrassing scandal. And it appears that it was about to get much worse for Tressel, as Sports Illustrated reports that it had informed Ohio State officials Saturday of a pending investigation showing that the violations at Ohio State were much broader and went on for much longer than the university has acknowledged. In a videotaped statement  Monday, Gene Smith, the athletics director, said that Tressel had emerged from a discussion between the two Sunday night persuaded that resigning was in his and Ohio State's best interests.