"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.
Officials with the U.S. 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.