A little-used but effective 18th-century law on rebellion against the government is helping the United States in its investigation into and prosecution for the Sept. 11 attacks.

The statute on sedition  conspiring to wage war or overthrow a government  evolved from laws dating back more than two centuries. It allows federal authorities significant flexibility in preparing cases against people who plot but don't execute crimes against the U.S.

The government suggested its approach in a perjury indictment last week.

The federal grand jury that brought the case against an associate of two of the hijackers is investigating "seditious conspiracy to levy war against the United States," the indictment states.

One expert said that federal prosecutors "appear to be right on the money" in using the sedition law to address possible terrorist collaborators.

"To the extent a jihad (holy war) is invoked against the United States, it's like an announcement that 'I'm putting myself under this statute,'" George Washington University law professor Stephen Saltzburg said.

The law reads:

If two or more persons in any state or territory, or in any place subject to the jurisdiction of the United States, conspire to overthrow, put down or to destroy by force the government of the United States, or to levy war against them, or to oppose by force the authority thereof, or by force to prevent, hinder or delay the execution of any law of the United States or by force to seize, take or possess any property of the United States contrary to the authority thereof, they shall each be fined under this title or imprisoned not more than 20 years, or both.

Prosecutors used the seditious conspiracy law to win convictions in a case against a Muslim cleric and co-defendants who planned to blow up the United Nations.

Chicago attorney Jeremy Margolis successfully prosecuted four Puerto Rican nationalists for seditious conspiracy in the 1980s for planning to bomb a Marine training center and an Army Reserve facility.

The object of the conspiracy was to change the policies of the U.S. government "as opposed to doing a particular criminal act  blow that up, take that down, shoot that person," Margolis recalled.

Law enforcement officials, speaking only on condition of anonymity, said prosecutors are examining other cases in which they might use the sedition law against people who did not carry out attacks but had been in various stages of planning.

The law imposes up to 20-year prison terms when two or more people "conspire to overthrow, put down, or to destroy by force the government of the United States, or to levy war against them."

The U.S. law on sedition dates back to the 1790s when the Alien and Sedition acts of the John Adams administration targeted people who criticized the government. The acts expired and were not renewed amid a storm of criticism.

A new law passed during the Civil War served as the basis for the current statute.

There were Confederate sympathizers in the North and the law was passed to make it easier to punish people who conspired against the union, said University of Michigan law professor Richard Friedman.

The government used the sedition law after World War I to convict anarchists. In the 1950s, the Supreme Court upheld convictions of communists on sedition charges for teaching doctrines that were held to be subversive.

"These weren't people blowing things up; they were basically basement seminars where people would read Marx," said constitutional law professor Richard Primus of the University of Michigan.

"Teaching people that the government is bad in the abstract is a constitutional right, but once you go beyond to an agreement to commit crimes, that becomes clearly punishable," said UCLA law professor Eugene Volokh.

The Associated Press contributed to this report.