WASHINGTON - The top federal prosecutor in New Orleans, the longest-serving U.S. attorney in the country, resigned Thursday amid an investigation into possible prosecutorial misconduct by two of his top deputies.

Jim Letten, the U.S. attorney for the Eastern District of Louisiana, announced his resignation at a news conference. In an emotional speech, he praised the office and said he would stay on briefly to help with the transition to new leadership.

The resignation comes eight months into a scandal that led to a Justice Department investigation of Letten's top deputy and a second veteran prosecutor in connection with anonymous online criticism of the target of a federal inquiry.

Jan Mann, former first assistant U.S. attorney, and Sal Perricone, former assistant U.S. attorney, acknowledged using aliases to post comments on the Times-Picayune website that were highly critical of the owner of a landfill that was under federal investigation, according to court papers. Perricone, a member of Letten's inner circle, resigned and Mann was demoted.

The Justice Department's Office of Professional Responsibility is investigating the episode, but last week a federal judge increased the pressure on Letten by calling for an independent counsel.

The judge, Kurt Engelhardt, also called for the department to investigate leaks of grand jury information to the media by prosecutors in the Danziger Bridge case in which New Orleans police officers shot innocent residents after Hurricane Katrina and then tried to cover it up.

In a statement Thursday, Attorney General Eric H. Holder praised Letten's service and announced the appointment of an interim U.S. attorney, Dana Boente of the Eastern District of Virginia. Through a spokeswoman, Holder declined to comment on the scandal.

The online scandal came to light in March when Fred Heebe, the owner of the landfill under investigation, filed a defamation lawsuit. He had been attacked repeatedly by an anonymous critic on the newspaper website and he hired a former FBI agent to find his critic.

Perricone admitted posting the derogatory information and making similar attacks on attorneys, defendants, police officials, and judges.