WASHINGTON -- Congressional Democrats turned up the pressure on the Obama administration Tuesday to start a criminal investigation by a special counsel into harsh interrogations of terrorism suspects.

It would be a conflict of interest for President Obama's Justice Department to investigate lawyers from the Bush administration, even though they no longer work for the government, Democrats on the House Judiciary Committee said.

In a letter to Attorney General Eric Holder, the Democrats wrote, "It is impossible to determine at this stage, and before conclusion of the necessary investigation, whether additional conflicts of interest might exist or arise."

The letter said a special counsel's investigation would insulate the department from accusations that the investigation was politically inspired.

Most of the 16 signers were Democratic liberals. Seven committee Democrats did not sign the letter, nor did any of the 16 Republicans.

Justice Department spokesman Dean Boyd said the letter will be reviewed, but he pointed out the department's Office of Professional Responsibility is investigating whether the memos are consistent with professional standards required of department lawyers.

"As with any issue, we will follow the facts where they lead," Boyd said. "As the attorney general has said, it would be unfair to prosecute dedicated men and women working to protect America for conduct that was sanctioned in advance by the Justice Department."

Obama has said it would be up to Holder to determine whether "those who formulated those legal decisions" should be prosecuted. The methods, described in the Bush-era memos, included slamming detainees against walls and subjecting them to simulated drowning known as waterboarding.

The president said he would not seek to punish CIA officers and others who carried out interrogations.

Sen. Carl Levin, D-Mich., chairman of the Senate Armed Services Committee, also has proposed that independent investigators determine whether Bush administration officials ought to face charges.

Conservative Republican senators have characterized the Democratic effort as counterproductive and politically motivated at the least and, at worst, damaging to national security.

Levin recommended that the Justice Department select up to three people outside the department, such as retired federal judges, to recommend any charges or other actions against lawyers and others who developed the policies.

In addition to lawmakers, a coalition of liberal groups delivered petitions to Holder demanding that he name an independent counsel. The groups included the American Civil Liberties Union, MoveOn.org, and the Center or Constitutional Rights.

The ACLU obtained four of the Justice Department memos that provided the legal framework for the interrogation policies.