A vague threat apparently posted by an Australian on two social networking sites led to the lockdown on Wednesday of all schools in the busy midwestern US city of Minneapolis, officials said.
City officials received a phone call early Wednesday alerting them that a young man planned to "shoot up" an unspecified school and then kill himself, police said.
Officials decided to place all schools on a "code yellow" lockdown as a precautionary measure while investigating the validity of the threat.
The lockdown, which affected around 34,000 students at 60 schools in the Minneapolis public school system, meant children were kept in their classrooms while outside visitors were strictly limited.
"We took this very seriously," Minneapolis police spokesman Jesse Garcia told reporters.
"We err on the side of caution," added Minneapolis Public Schools spokesman Stan Alleyne.
"The schools operated as normally as possible but we just had more security."
The unprecedented measure was lifted at the end of the school day and busing proceeded as normal, school officials said.
Meanwhile, in Minneapolis's twin city of St. Paul, the Cretin-Derham Hall Catholic school was also locked down after it received an e-mail notification of the threat.
"It was determined that the source of the threats was the same," St. Paul police Paul Schnell told AFP.
"The source of the threat originated out of Australia."
Schnell said authorities were still trying to determine who was responsible for the threat and did not identify the social networking sites involved.Reuse content