A bomb threat called in at Westfield High in Chantilly on Tuesday afternoon forced the school into lockdown while police swept the building, the only unusual incident in an otherwise normal first day of classes in Fairfax County.

Principal Tim Thomas wrote a letter to parents after the incident to report that the call was found to be a false alarm.

“We have no reason to believe that our students or staff were in any danger,” Thomas wrote. “The lockdown was a precautionary step.”

Thomas wrote that school was placed on lockdown at approximately 3:30 p.m, after classes were dismissed for the day. Thomas noted that about 200 students and staff were still inside at the time of the lockdown while police searched the building. Most of the students and staff were in the process of being released later Tuesday afternoon, Thomas wrote.

The bomb threat marred an otherwise smooth opening for the first day of schools in Fairfax, where 186,785 students are expected to enroll this fall.