WASHINGTON - Attorney General Eric H. Holder Jr. said Tuesday that he had ordered the Justice Department and the FBI to open an investigation into whether Internal Revenue Service officials broke any criminal laws by singling out conservative groups for special scrutiny.

The activities of IRS officials are already the subject of an investigation by the agency's inspector general. The results of that inquiry, which are expected in the next several days, are likely to detail how officials at the agency selected political groups for extra scrutiny about their tax status.

Speaking at a news conference called Tuesday to discuss Medicare fraud, Holder said that he had ordered a second investigation to determine whether any criminal laws may have been broken by the officials at the tax collection agency.

The attorney general said there were "a variety of statutes within the IRS code" that could be the basis of a criminal violation. He said officials conducting the investigation would also look at "other things in Title 18" of the U.S. Code. Title 18 is the overall criminal code for the federal government.

Holder also fielded questions about the seizure of telephone records from reporters and editors at The Associated Press, which apparently came in connection with an investigation of leaks inside the executive branch.

Holder said that he had recused himself last year from the leak investigation and therefore had not made the decision to seek sweeping subpoenas for two months of call records for 20 telephone lines used by the AP and its journalists. He said he decided to turn over supervision of leak inquiries to his deputy, James M. Cole, "to make sure that this investigation was seen as independent" after FBI agents interviewed him about leaks in June.

But Holder said that the leak in question - the revelation by the AP of a foiled terrorist plot by al-Qaida's branch in Yemen a year ago - was among the two or three most serious leaks he had seen since the 1970s.

"It put the American people at risk," he said, without elaborating.

Holder said he was confident that his subordinates had sought the subpoenas in accord with Justice Department regulations.