NIWOT, Colo. (CBS4) – Niwot High School students will return to class Monday after a threat caused the school to shut down on Friday.

The closure came after students found a hit list with the names of 30 students. The threats were not detailed.

Bomb squads and bomb-sniffing dogs searched the school. They didn’t find anything, but the criminal investigation continues.

Detectives have interviewed persons of interest but no arrests have been made. One student has been suspended.

(credit: CBS)

The school will have an increased security presence this week.

“Additional school staff will also be on campus this week and district Operations and Maintenance technicians inspected and enhanced all of the building’s existing security systems,” Principal Dennis Daly wrote in a letter to parents over the weekend.