SIOUX CITY | Police and school district officials are searching for the person or persons who disrupted classes at Sioux City North High School Tuesday, with a bomb threat aired via an online video game.

The high school was quickly evacuated, with the approximately 1,500 students exiting the rear of the building, where they boarded buses. The buses then made the short trip down Outer Drive North to the adjacent North Middle School, where the high school students spent a few hours in the gymnasium.

"It was an incredible disruption to the day," school superintendent Paul Gausman said at an afternoon press conference, about an hour after students were shuttled back to North High about 2 p.m.

Police responded to the bomb threat just before 10 a.m. at the school in the 4200 block of Cheyenne Boulevard. Police made a sweep of the school at that point, while district officials began implementing a crisis plan to address the threat.

School district spokeswoman Mandie Mayo said the threat was first identified by a student who saw the message displayed on an electronic message board for the Minecraft, a video game in which players place blocks and go on adventures.

The message said the bomb would detonate at 1:28 p.m. After that designated time passed without an explosion, police conducted a second sweep of the high school but didn't find a device, Lt. Pat McCann said.

Gausman said parents were notified about the threat some time after students were transported to North Middle School. In an era of social media and mobile phones, some parents may have first heard about the incident from their own children, he acknowledged.

"Some (parents) would have been frustrated with that," the superintendent said, adding that it was paramount to get the students safe first.

The students returned to the building about an hour before the normal 2:55 p.m. end of the school day.

Gausman said school officials are working with police to find any culprits associated with the threat. Mayo said it is more likely that one person, rather than several, logged into the Minecraft account to place the threat.

"We are doing everything we can to determine the person who made this threat," Gausman said.

Criminal charges likely would be filed, and if the culprit is a student, he or she also would face a range of punishments from the district, including expulsion.