WASHINGTON - Citing time pressures, former Senate Majority Leader George Mitchell said Wednesday he would not serve on an independent panel investigating the Sept. 11 attacks, and former House Foreign Affairs chairman Lee Hamilton was named to take his place.

Mitchell, a former Democratic senator from Maine, agreed two weeks ago to serve as vice chairman of the commission to be headed by former Secretary of State Henry Kissinger.

The commission is charged with investigating possible intelligence, aviation security, immigration or other policy lapses related to the 9-11 attacks.

Mitchell said he had decided he could not take on the part-time but demanding position while also juggling his responsibilities at his Washington law firm job.

Mitchell said in a letter to Democratic congressional leaders he was declining the post reluctantly but said it would "require more time than I anticipated and more than I now can commit to." The 10-person panel has not yet begun work.

He also dismissed any suggestion that there were conflicts of interest between the commission and his legal work.

Since leaving the Senate, Mitchell has taken on high profile peace missions in Northern Ireland and the Middle East.

Kissinger's selection by President Bush sparked controversy, both because of his policy-making role during the Vietnam war and the bombing of Cambodia, and because he is now a high-priced private international consultant. He has stated publicly that there are no conflicts between the commission's work and clients at his New York-based consulting service.

The commission was established in the recently enacted law that created the new Department of Homeland Security.

House Democratic Leader Dick Gephardt of Missouri and Senate Democratic Leader Tom Daschle of South Dakota said they had asked Hamilton, an Indiana Democrat, to be vice chair instead of Mitchell.

Hamilton, a respected voice on foreign policy, played a key role in investigating covert arms sales to Iran in the 1980s and is now the director of the Woodrow Wilson International Center for Scholars.

The other Democratic appointees are Sen. Max Cleland of Georgia, a wounded Vietnam vet who was defeated in the November elections that made homeland security an issue; retiring Rep. Tim Roemer of Indiana, an early sponsor of legislation calling for a 9-11 commission; Richard Ben-Veniste, a Washington lawyer and former Watergate investigator; and Jamie Gorelick, vice chair of Fannie Mae and former deputy attorney general under former President Bill Clinton. All have some defense or intelligence background.

Earlier this week Republicans named former Sen. Slade Gorton of Washington state as one of their appointees. The rest must be named by Dec. 15.