The Federal Bureau of Investigation is struggling to find enough agents and resources to investigate criminal wrongdoing tied to the country's economic crisis, according to current and former bureau officials. The bureau slashed its criminal investigative work force to expand its national security role after the Sept. 11 attacks, shifting more than 1,800 agents, or nearly one-third of all agents in criminal programs, to terrorism and intelligence duties. The cutbacks have left the bureau seriously exposed in investigating areas like white-collar crime, which has taken on urgent importance in recent weeks because of the nation's economic woes. So depleted are the ranks of the F.B.I.'s white-collar investigators that executives in the private sector say they have had difficulty attracting the bureau's attention in cases involving possible frauds of millions of dollars. Since 2004, F.B.I. officials have warned that mortgage fraud posed a looming threat, and the bureau has repeatedly asked the Bush administration for more money to replenish the ranks of agents handling nonterrorism investigations. But each year, the requests have been denied, with no new agents approved for financial crimes, as policy makers focused on counterterrorism. According to previously undisclosed internal F.B.I. data, the cutbacks have been particularly severe in staffing for investigations into white-collar crimes like mortgage fraud, with a loss of 625 agents, or 36 percent of its 2001 levels.