Palo Alto High School students went under shelter-in-place Tuesday morning after a teacher found a typed note on a classroom door threatening to “shoot up” the classroom.

Paly administrators reported the incident to Palo Alto’s 24-hour dispatch center at around 8:57 a.m., and the campus was put into a shelter-in-place status soon after. Classes continued as normal, and the shelter-in-place was lifted around 10:30 a.m. The investigation is ongoing and the person responsible for leaving the note may face criminal charges, the Palo Alto Police Department wrote in a press release.

“The school is maintaining a normal class schedule,” Palo Alto Unified School District Superintendent Don Austin said in an email. “Many classes are experiencing exams this week, and the district is keeping safety as a priority. Authorities will remain on campus throughout the day.”

The Palo Alto Police Department encourages those with information about the incident to call their 24-hour dispatch center at (650) 329-2413. Anonymous tips can be emailed to paloalto@tipnow.org or sent via text message or voice mail to (650) 383-8984.