AUSTIN — Murchison Middle School in northwest Austin was placed on lockdown Thursday morning after the principal said a legally-armed parent -- who is also an officer -- walked through the school, according to a letter sent to parents.

At about 11:15 a.m., principal Brad Clark said he received a report that a man in plain clothes wearing a firearm was walking around the school. Clark immediately placed the school on lockdown, and police arrived to investigate.

It was later discovered that the man was a parent there to attend a parent-teacher conference who had properly checked in at the front desk. The school district confirmed that the parent is a law enforcement officer from another jurisdiction. However, the principal said "neither me, security, nor our school resource officer was made aware of the presence of a legally armed person in our building."

Clark said the school is now working on a system to notify all necessary personnel in the event of a legally armed visitor to Murchison Middle School.

Under federal law, K-12 schools are designated gun-free zones. But federal and state law -- as well as AISD policy -- make exceptions for law enforcement officers, both on- and off-duty.

Students described to KVUE the moments after their school was placed on lockdown.

"We all went down under the windows and we barricaded the door, and everybody was freaked out," sixth-grader Colt said. "I assumed it was just like the regular, regular, regular drill. But I didn't realize that it was an actual person in the building."

Another student named Simon told KVUE he didn't hear the principal too well over the intercom, so he was unaware the lockdown was real.

"We didn't know that it was, like, a false alarm, yet, and it was, like, really scary. And we thought, like, 'Oh, my God,'" he said.

All students and staff at the school are safe and the school is operating under normal procedures.