HARRISBURG, PA - The federal government has placed the Three Mile Island nuclear power plant located near Harrisburg on high alert due to what has been termed a "credible threat" on the facility.

No aircraft were allowed near the plant for a four-hour period initially as the military and federal and state law enforcement officers patrolled the area. The local airports were then opened again for business.

Heightened security will remain in place, officials said. Since September 11, security had already been increased.

The plant’s Unit 2 was the site of a major nuclear accident in 1979.