The allegations prompted denials from governments in the former Soviet bloc. Such prisons, European officials say, would violate the continent's human rights principles.
"If the leadership determines that we should investigate the leak, it would be much like the 9/11" commission, Roberts said. The Intelligence Committee chairman did not dispute a reporter's suggestion that a probe would raise First Amendment press-freedom issues.
Such an investigation would become "very difficult when you're getting into matters like this," the senator said.
Roberts also said he would support hearings into the importance of maintaining a covert agent's cover, a topic triggered by the leak of Plame's identity, eight days after her husband accused the Bush administration of manipulating prewar intelligence to exaggerate the Iraq threat.
Democratic leader Harry Reid of Nevada said the House and Senate committees with normal jurisdiction should conduct any hearings, not a bicameral committee as suggested in the letter of the two Republican leaders.
House Minority Leader Nancy Pelosi, D-Calif., said any such joint investigation should also investigate possible manipulation of prewar intelligence on Iraq.
"If Speaker Hastert and Majority Leader Frist are finally ready to join Democrats' demands for an investigation of possible abuses of classified information, they must direct the House and Senate Intelligence Committees to investigate all aspects of that issue," Pelosi said.
Kenneth Roth, executive director of Human Rights Watch, said Republicans "should be focused on the illegality of these prisons, not the revelation of the illegality."
Associated Press Writer Pete Yost contributed to this report.