The Bush administration may have acted illegally when former vice-president Dick Cheney kept Congress in the dark about a CIA counter-terrorism program, according to two senators.

Cheney's decision was "something that should never, ever happen again," Sen. Dianne Feinstein, the chairperson of the U.S. Senate Intelligence Committee, told Fox News on Sunday.

Democratic Sen. Dick Durbin, D-Ill., told ABC's This Week that Congress should investigate the matter.

"It could be illegal," he said.

The unidentified program began shortly after the Sept. 11, 2001 terrorist attacks. It remains steeped in mystery, although the program reportedly did not involve domestic activities.

Cheney ordered the CIA to conceal the program from Congress, according to The New York Times. It remained secret for eight years, until current CIA director Leon Panetta learned of its existence last month.

Panetta's subordinates told him about the program on June 23, and he briefed two intelligence committees the following day, the Times reported.

Under the National Security Act of 1947, the U.S. president must ensure the intelligence committees are informed about "intelligence activities in the United States, including any significant anticipated intelligence activity."

However, the Act also stipulates briefings should be held "to the extent consistent with due regard for the protection from unauthorized disclosure of classified information relating to sensitive intelligence sources and methods or other exceptionally sensitive matters."

Feinstein said the law shows Congress should have been told about the program, and the Bush administration's failure to do so "is a big problem."

It's unclear how U.S. President Barack Obama will address the issue. Since taking office, he has been reluctant to uncover any alleged wrongdoing by the Bush administration.

When some Democrats pressed for a "truth commission" to examine such allegations, Obama said the U.S. should be "looking forward and not backwards."

However, Ivan Eland, a defense analyst from the Independent Institute in Washington, said the program should be probed as it must have been "very serious."

"It certainly is very interesting that it (only) took Panetta a day to close it down and report it to Congress," Eland told CTV News Channel Sunday.

He added that in recent years, the U.S. has "gone awry in its counterterrorism activities," and it's now time to reign in those executive powers.

"We've seen this grandiose expansion of executive power way beyond what the U.S. founders wanted," Eland said.

"That makes for very bad policy when there are no checks and balances, because that's the way the U.S. system works."

Eland stressed that making changes will also impact U.S. allies, as Washington's security policy "sort of drags Canada along with it ... since we share a long border."

"It's probably important for Canada (and) the U.S. to get to the bottom of this."

With files from The Associated Press