Police in Fairfax County said West Potomac High School was on lockdown Friday after it received an anonymous threat that did not appear to be credible.

It was not immediately clear what the threat involved. In several Twitter messages, Fairfax County Police and the Fairfax County School system said the threat "doesn't appear credible" to the school, which is in the Alexandria area.

Law enforcement officers were at the school and police said, "students are safe."

The lockdown was lifted at around 11:15 a.m.