The White House is seriously considering a proposal to cede the FBI's counterterrorism function to a new agency similar to MI5, the British domestic spy agency. The idea scares almost everyone who has actually been involved in investigating terrorism, and with good reason.

A domestic spy agency would mean a return to the years when the FBI, under J. Edgar Hoover, investigated citizens simply for subscribing to leftist publications or for speaking out against the government. It was all done in the name of intelligence gathering, an amorphous standard that could be used to justify investigating and compiling files on anyone perceived to be different. In the process, the FBI -- often with the approval of presidents -- not only violated Americans' rights under the Constitution, but also lost sight of what it was supposed to be uncovering. Because Hoover confused political dissent with spying, the FBI did a poor job of investigating the real threat at the time: espionage.

While the FBI has many serious problems, the fact is that in a six-year period before 9/11, the FBI stopped 40 terrorist plots that would have led to the killing of tens of thousands of people. Since 9/11, the FBI has brought charges against 200 suspected terrorists. With the help of the CIA, it has rolled up nearly 100 plots worldwide before they happened.

Over the years the FBI has developed informants to take on the Mafia, white-collar criminals, domestic terrorists, corrupt public officials and spies. In doing so, it has wiped out entire criminal organizations. What has made the FBI effective and kept it from engaging in the abuses of the Hoover years is that, under the supervision of the Justice Department, it focuses on violations of criminal laws. If that standard for undertaking investigations is removed, investigators lose their compass, straying into extraneous matters such as political beliefs and associations and forgetting what their real target is.

If a nexus to pursuing criminal violations keeps investigators on course, it also provides leads and tools to conduct terrorist investigations. Counterterrorist investigations often begin with probes of other crimes -- money laundering, fraud, drug violations. As the FBI is now constituted, those leads can be passed along seamlessly to agents pursuing terrorist activities.

Who would staff a new domestic intelligence-gathering agency? Only FBI agents have the training and experience to conduct such sensitive investigations lawfully. Who would want to work for a new, untested agency? For all its problems, the FBI is known as the premier law enforcement organization in the world. It therefore attracts the very best applicants. How would the new agency use the coercive tools of law enforcement to get people to talk? It would have to call in the FBI. Thus, a domestic intelligence agency would create yet more walls and generate more turf battles within the counterterrorism effort. It would be, as FBI Director Robert S. Mueller III said recently in New York, "a step backward in the war on terror, not a step forward."

As it happens, MI5 has not had a particularly impressive record in uncovering terrorists, whether from the Irish Republican Army or al Qaeda. Nor do most Americans want to adopt the British model when it comes to protecting individual rights.

"MI5 looks with a bit of envy at our system, where we have a law enforcement organization that also gathers intelligence," said John L. Martin, who dealt extensively with MI5 for 25 years as chief of the Justice Department's counterespionage section. "Because it has no law enforcement powers, MI5 has the usual problems with sharing intelligence information leading to arrests and prosecutions."

The proposal to create a separate counterterrorism agency is an effort to come up with a quick fix. In fact, it would create a nightmare agency, one that would neither know how to investigate terrorism nor be equipped with the inherent safeguards necessary to protect American freedoms.

The writer, a former Washington Post and Wall Street Journal reporter, is author of "The Bureau: The Secret History of the FBI."