The government is taking the threats seriously, though officials have preliminarily determined that the information is not credible enough to act upon, said a government official familiar with the investigation.
The official, who spoke on condition of anonymity, said the alleged plot to attack on America's celebration of independence is one of scores of threats filtering through U.S. intelligence and is not considered serious enough to formally warn the American public or change the nuclear industry's already high level of alert.
"We will continue to assess the information, however," the source said.
The threat received last week suggested that an unidentified Islamic terrorist group is planning to attack the Three Mile Island nuclear facility in Pennsylvania or another plant elsewhere in the Northeast, the source said.
The Washington Times first reported the threat.