As debate over natural gas drilling in the Marcellus shale reaches a fever pitch, state and federal authorities are warning Pennsylvania law enforcement that "environmental extremists" pose an increasing threat to security and to the energy sector.

A confidential intelligence bulletin sent from the Pennsylvania Department of Homeland Security to law enforcement professionals in late August says drilling opponents have been targeting the energy industry with increasing frequency and that the severity of crimes has increased.

It warns of "the use of tactics to try to intimidate companies into making policy decisions deemed appropriate by extremists," and states that the FBI -- the source of some of the language in the Pennsylvania bulletin -- has "medium confidence" in the assessment. A spokesman for the FBI did not immediately respond to a request for comment.

The advisory, a copy of which was obtained by ProPublica, doesn't cite the specific incidents causing concern. It is also unclear from accounts from state law enforcement officials whether the incidents in Pennsylvania posed a substantial threat, or what effect the advisory might have on public gathering and the debate over drilling in the state.

Pennsylvania State Police said there have been only a few isolated crimes involving drilling facilities.

"We haven't had any incidents of any significance to date where we have identified a problem, or any environmental extremists," said Joseph Elias, a captain with the Pennsylvania State Police Domestic Security Division, which was not involved in issuing the bulletin.

An aide to Gov. Ed Rendell -- speaking on behalf of the state's Homeland Security Office -- said the advisory was based on five recent vandalism incidents at drilling facilities, including two in which a shotgun was reportedly fired at a gas facility.

"All this security bulletin does is raise awareness of local officials. It doesn't accuse anyone of local activity," said the spokesman, Gary Tuma. "Where the professionals detect a pattern that may pose a threat to public safety, they have a responsibility to alert local law enforcement authorities and potential victims."

Anti-drilling activists in the state say that public hearings and other events have been peaceful and that they see no evidence of violent opposition. Given the lack of evidence about "extremist" crimes, they say, the bulletin casts drilling opponents as criminals and threatens to stifle open debate.

"It may very well be designed to chill peoples' very legitimate participation in public decision making," said Deborah Goldberg, an attorney with Earthjustice, a national group pressing for stronger environmental protections. "If people who have concerns fear that they are going to be treated as a security threat they may very well be afraid to go and express their views."

The advisory lists a series of public hearings on drilling permit issues across the state as potential flash points. It also mentions a Sept. 3 screening of the anti-drilling film "Gasland" in Philadelphia that went off without incident. Language describes "environmental activists and militants" on one side of the debate and "property owners, mining and drilling companies" on the other.

Finally, the bulletin groups the public hearings and film screening with protest rallies for anarchist clubs focused on "evading law enforcement," and with a Muslim advocacy group's rally for the release of suspects in an alleged terror plot at Fort Dix, N.J.

The advisory was sent to state law enforcement and local government groups, as well as businesses with a specific concern addressed in the bulletin. It was not intended to be distributed to the public.

In issuing such an advisory, the government has to walk a fine line between the need to respect the fundamental rights of freedom of speech and the need to keep the public safe, said Nathan Sales, an assistant law professor at George Mason University and a former policy development staffer at the U.S. Department of Homeland Security.

"The question is how to accomplish the one with minimal consequences to the other," he said.

A pro-drilling group, the Marcellus Shale Coalition, characterized the vandalism in Pennsylvania as "directed at preventing our industry from safely delivering these resources to Pennsylvanians."

The group's president, Kathryn Klaber, said she supported civil debate over drilling, "but to the extent they go in the other direction, and potentially devolve in a manner that undermines our ability to keep our folks safe, then we will have a problem," she said.