Americans are under siege by terrorists right now, but the primary source of the problem is not al Qaeda or any other foreign terrorist organization. The threat stems from home-grown activist groups with names like: the Animal Liberation Front, the Earth Liberation Front, and Stop Huntingdon Animal Cruelty. And it’s not just left-wing radicals who are causing the problems. Domestic terror threats include bombings by right-wing terrorists such as Timothy McVeigh and abortion clinic bomber Eric Rudolph.

While we still need to continue to address the threat of international terrorism, we also have to start working to minimize the harm done by domestic terrorists as well. This need to shift priorities was confirmed when an FBI official told lawmakers at a recent Senate hearing that the nation’s top terrorist threat comes not from abroad but from these radical activist groups raised right here at home. The FBI’s deputy assistant director for counter-terrorism warned that, in terms of the number of crimes they commit and the overall rate of property damage they cause, the danger posed by those groups far outweighs any threat from international terrorism.

Take the group called Stop Huntingdon Animal Cruelty as just one example. Known as SHAC, it was founded in Great Britain in 1999. Its aim is to rescue animals used in medical and scientific research and shut down businesses that rely on their use.

Its original target was Huntingdon Life Sciences, or HLS, a company that uses animals in its research to develop treatments for diseases such as cancer and AIDS. In February 2001, members of SHAC attacked the company’s chief executive outside his home in England and beat him with baseball bats and axe handles. Several months later, HLS’s marketing director was temporarily blinded by an attack of chemical spray.

It was amid this air of escalating violence in both rhetoric and tactics that the U.S. Senate held its Environment and Public Works Committee hearings in May. FBI testimony indicated that it had 150 open investigations into such activities, and that some 1,200 crimes between 1990 and 2004 had been publicly claimed by activist groups. These crimes include arson, bombings, theft, animal releases, vandalism, harassing phone calls, letters rigged with razor blades, and office takeovers.

In May, animal rights activists stole a credit card from...