Emilio T. Gonzalez, a Cuban exile appointed by the Bush administration to modernize the besieged federal agency that approves green cards and citizenship for immigrants, announced Thursday he is stepping down after two years as its director to return to Miami.

Gonzalez, 51, obtained mixed results in his efforts to adopt new technology, overhaul services for immigrants and expand the workforce at U.S. Immigration and Citizenship Services, which has been beset with severe backlogs since Sept. 11. While he earned praise from advocates for accessibility and responsiveness, his tenure was marked by a sharp increase in fees charged to immigrants, a step Gonzalez said was necessary to finance badly needed improvements at the agency, including hiring workers to reduce backlogs. The agency is funded almost exclusively by the fees.

Critics say Gonzalez left the agency unprepared for a surge in citizenship applications that resulted when 460,000 immigrants filed paperwork to beat the start of the higher fees last July — more than six times the number who applied the previous July. The average processing time, which had been reduced to seven months in the early part of Gonzalez' tenure, ballooned to 18 months. At the end of last year, the number of pending cases stood at almost one million. In a statement Thursday, U.S. Sen. Edward Kennedy, D-Mass., said the naturalization system is ''in crisis,'' noting that many of those who applied for naturalization in the hope of voting in November's presidential election won't become citizens in time.

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