"If someone wants to believe in the bizarre, well, that's not a violation of law," Special Agent Frank Scafidi said.
"Absent any conspiracy to commit a crime, there is nothing we can do."
State and federal authorities admit to growing anxiety about such cults following the arrest of 14 members of the Denver-based "Concerned Christians" in Israel. At least 78 members of the group have disappeared in recent months, apparently to follow leader Monte Kim Miller, who has predicted he will die Dec. 31 on the streets of Jerusalem.
Israeli police raided two suburban Jerusalem homes Sunday reportedly after several weeks of surveillance of members of the cult. Authorities confiscated boxes of documents and detained eight adults and six children thought to be plotting a gunbattle with police to trigger the second coming of Christ.
Officials with the Justice and State Departments say they have no plans to increase security measures against religious cults.
"We are not putting our heads in the sand," Scafidi said.
"We are aware of the importance of this date coming up. We have been consulting with psychologists and experts in this field so that, if the need arrives, we can deal with it."
He said FBI agents trained to negotiate in hostage and armed standoff confrontations have rethought their tactics following the 1993 debacle at the Branch Davidian cult's compound near Waco, Texas. A 51-day siege of followers of David Koresh ended in the deaths of more than 80 people after cult members set fire to their fortress-like structure when federal agents tried to storm the building.
But the bureau is taking no other action.
"We know there are groups that sit around in caves amongst themselves and plan the end of the world," Scafidi said. "That's fine, just as long as they don't try to do something to bring the end to fruition."