Shortly after the London bombings, it became fashionable for some American commentators to cite the alienation of British Muslims as an example of a massive failure of assimilation -- a state of affairs, they asserted, that is inconceivable in the United States.

Americanization, went their argument, has virtually eliminated teeming groups of disaffected Islamic immigrants within our shores.

Well, that may be true but it's not the whole truth concerning American Muslims.

Islam in the United States is not solely the province of immigrants from North Africa, the Middle East and South Asia. The largest and fastest-growing ethnic group of American Muslims is African Americans, whose estimated numbers range between 1.3 million and 2 million. Most, by the way, are Sunni Muslims and not followers of Minister Louis Farrakhan's racially exclusive Nation of Islam.

What's more, the group within the African American Muslim community that is experiencing the most explosive growth is probably the least assimilated: black inmates. Good statistics are hard to come by, but one estimate places the number of Muslim converts in prison above 250,000. What brings them to Islam? Survival? Acceptance? Rejection of Christianity? Spiritual transformation?

One thing for sure: It's not Americanization.

Which gets us to the FBI, converts to Islam and a possible terrorist plot in California.

When FBI director Robert S. Mueller III joined us at The Post for lunch in June 2002, Muslim converts in prison seemed to be the last folks on his mind. At the time, Mueller was preoccupied with discussing the bureau's new post-9/11 mandate to detect and foil terrorist actions against American targets before they happened.

Not so today.

Mueller recently told Congress that one area of the war on terrorism that causes him great concern is the potential for extremist groups such as al Qaeda to recruit radicalized American Muslim converts. Mueller drew a bead on the American prison system, which he described in written testimony as "fertile ground for extremists who exploit both a prisoner's conversion to Islam while still in prison, as well as their socioeconomic status and placement in the community upon their release."

That concern is no longer theoretical.

This week brought news that three California men are currently being investigated as part of a possible plot to launch assaults against National Guard facilities this Sept. 11 and against Jewish targets on Yom Kippur.

One of them, 25-year-old Levar Haney Washington, had served time at the California State Prison in Folsom and, during his incarceration, converted to a radical Islamic group known as Jamiyyat Ul Islam Is Saheeh. Gregory Vernon Patterson, 21, arrested with Washington on unrelated robbery charges, recently converted to Islam. The third man in custody, Hammad Riaz Samana, 21, is a Los Angeles resident. Washington and Patterson are African American; Samana is a Pakistani national.

Authorities, according to news reports out of California, are trying to link the three men to Peter Martinez, 36, and Kevin Lamar James, 29, two state prison inmates and members of the same radical Islamic group. Martinez and James reportedly had recruited other inmates to join in a "jihad against the United States."

There's talk that this case could represent a new U.S. front in the war against terrorism: homegrown terrorists among prisoners and former inmates with ties to Islamic extremists. They cite the case of Jose Padilla, the ex-gang member from Chicago now held without charges as a suspected terrorist. He converted to Islam while in jail. Could those in California be the latest incarnation?

In the fullness of time, maybe we'll know more.

Yesterday the Council on American-Islamic Relations of Southern California issued a statement urging the public "not to use this case to generalize and incriminate all Muslim inmates." It's a thought worth holding.

First, let's lay down a marker: Protecting the nation from terrorism must remain one of the government's highest priorities. Finding those who would aid or carry out attacks should top the list. And the effort must necessarily be intelligence-driven and proactive. Waiting until the bomb explodes is stupid. It's the government's duty to secure Americans at home and bring enemies to justice.

But we can't mangle the Constitution in the process.

Mueller, to his credit, assured us at lunch that his use of new investigative powers would remain behind, and not get ahead of, the law. That's also worth keeping in mind even as his agents work with law enforcement and corrections officials to defeat what he has termed "the recruitment and radicalization of prison inmates."

Thousands of inmates have converted to Islam; thousands more convert each year. Questions worth asking: How will authorities go about monitoring prison proselytizing? Who gets to decide when an American Muslim inmate is "radicalized"? How will the FBI staunch "recruitment"?

Recent American history justifies those questions and more.

Once upon a time the nation felt threatened. Fear of civil disturbances and unlawfulness by domestic groups reached the White House and Justice Department. In the name of protecting the nation, the FBI was mandated to "expose, disrupt, and otherwise neutralize" groups that posed a danger to national security. They called it COINTELPRO.

To make the program work within the African American community, the FBI created a "ghetto informant program" and recruited thousands of people to watch suspected militants in the black community. The FBI monitored bank accounts and trips, examined income tax returns and launched dirty operations to besmirch and "neutralize" so-called "Key Black Extremists." And it didn't take much to get labeled and investigated by the government.

Such tactics were unworthy of America then. They still are today. Even in a war against radical extremists -- in and out of jail -- to whom our Constitution means nothing.