A U.S. government bulletin is warning that violent extremists could try to use insider access to sabotage public utility facilities — including chemical plants, oil refineries and nuclear power plants.

This week's bulletin from the Department of Homeland Security notes that extremists have obtained insider positions and attempted to solicit employees at public utilities.

A Homeland Security spokesman, however, said the department has no credible evidence of an imminent threat. The spokesman said several recent incidents highlight the ongoing threat to infrastructure from insiders and outsiders seeking information that might be exploited in an attack.

The bulletin warned that such insider access could provide al-Qaida with an opportunity for an attack on or around the anniversary of the September 11, 2001, terror attacks.

Materials gathered in the U.S. raid that killed Osama bin Laden in Pakistan in May showed evidence that bin Laden wanted large-scale terror attacks on or around the anniversary.

The bulletin cites the case of an alleged al-Qaida recruit, an American, who had worked in five U.S. nuclear power plants in the eastern state of Pennsylvania before his arrest in 2010.