MINNEAPOLIS (WCCO) — Cretin-Derham Hall, a private high school in St. Paul, went on lockdown Thursday morning after school officials say a gunman was spotted nearby.

A note went out to parents via email shortly before 9 a.m. saying that the school was in lockdown after the gunman was reported in an alley off of Hamline Avenue. Police searched the area shortly after, and then issued an all-clear.

During the 30-or-so minute lockdown, students were inside locked classrooms, the school said.

Cretin-Derham Hall sits on Hamline Avenue, between Randolph and West Hartford avenues.

Police are investigating.