U.S. Attorney Mary Beth Buchanan vowed Friday to continue investigating executives at Mellon Financial Corp. after agreeing not to prosecute the company for destroying about 80,000 federal income tax forms and payments in 2001. "Somebody within Mellon directed the employees to destroy those tax documents," Buchanan said. "We want to know, How high within the corporation did this knowledge go?" Buchanan's comments came after the government and Mellon reached a settlement over the practice by overwhelmed employees at the Mellon Client Service Center, Downtown. Six former employees are awaiting trial on conspiracy charges. No trial date has been set. A seventh worker, Brian Metzer, 30, of Lawrenceville, pleaded guilty to theft of government property in January and agreed to help prosecutors.