WASHINGTON – Revelations about the Bush administration’s spying on U.S. soil have rekindled a debate that has simmered since Sept. 11, 2001: Should a new domestic intelligence service be created?

Intelligence experts who support creating an agency similar to Britain’s MI5 say it would centralize domestic spy functions under one roof and make it harder for the National Security Agency or Pentagon to secretly eavesdrop at home. Critics dismiss the idea as unnecessary.

The New York Times reported last week that the NSA has been monitoring telephone calls and e-mail messages domestically without obtaining court-approved warrants. Disclosure of the program followed reports that the Pentagon has been keeping tabs on anti-war protesters and others on U.S. soil.

“At what point do we need to think about what an MI5 would mean to the U.S.?” said Ronald Marks, a former CIA official who is now a senior fellow at George Washington University’s homeland security center. “The problem is that everybody’s doing a piece of (domestic spying) – the FBI, the Defense Department and now the NSA.”

President Bush has vigorously defended the NSA program as essential to fighting terrorism. Bush said it has been used only to monitor communications between the United States and other countries, not Americans talking to each other.

But University of Georgia political scientist Loch Johnson said such programs recall the 1970s, when disclosures about agencies conducting illegal wiretaps and other secret operations aimed at Americans led Congress to adopt a sweeping set of reforms. Johnson worked on a 1975 panel led by the late Sen. Frank Church, D-Idaho, which investigated the abuses.

“I have a lot of friends who were on the Church Committee, and we’re all rubbing our foreheads in disbelief,” said Johnson, who has been a consultant to the National Security Council and other federal agencies. “We’re wondering why people didn’t learn from that experience … We need to look much more closely at an MI5.”

The United Kingdom’s Security Service, historically known as MI5 – for Military Intelligence (Department) 5 – protects against foreign espionage and terrorism within the British Isles. It collects and analyzes intelligence, but unlike the FBI cannot arrest or detain suspects.

Although the Sept. 11 Commission considered recommending creation of a new MI5-like agency, it concluded the job should be left to the FBI, which Director Robert Mueller has reorganized to focus on counterterrorism. The creation of the new Office of the Director of National Intelligence was recommended by the commission as a mechanism to improve sharing of intelligence among agencies, not as a new source of it.

Some experts say the FBI, as a law enforcement agency, is ill-suited to the task of spying.

“The only reform issue that would improve the likelihood of anticipating a 9/11 would be creating an MI5 where you separate police and arrest powers from intelligence and collection powers,” said retired Army Lt. Gen. William Odom, NSA’s director from 1985 to 1988. “They’re antithetical activities – would you ask the Washington Redskins to win the American League baseball pennant?”

But other experts said a domestic spy agency would create more problems than it would solve.

Harry “Skip” Brandon, a former senior FBI counterterrorism official, says that because MI5 does not directly wield police powers, it has to hand off investigations to law enforcement agencies, which often have to start over in building legal cases against terrorists.

“Colleagues at MI5 have talked about this and said, ‘For God’s sake, don’t let them do it,’ ” Brandon said. “It strikes me that the answer is not forming new agencies. Let’s make the ones we have do the work they need to.”

Mark Lowenthal, a former CIA official who is now an intelligence consultant in Arlington, Va., said it would take considerable time to establish a new agency. “It’s not like we’re going to wave a wand and say, ‘Start behaving like MI5,’ ” Lowenthal said. “You have to think about how you’re going to do that – how to build a force, train it and then give it a legal charter in terms of permitted and non-permitted activities.”

Other experts said mechanisms are in place to restrain spying at home, most notably the court that issues warrants under the Foreign Intelligence Surveillance Act.

Reaction to the NSA program “is really much more of an argument about presidential power than about domestic intelligence,” said Gregory Treverton, a senior policy analyst at the Rand Corp., a national security think tank.

Civil liberties groups have feared it would be difficult to keep watch over a domestic spy agency. The Sept. 11 Commission said in its report that if such an agency were created outside the Justice Department, “the process of legal oversight – never easy – could become even more difficult.”