The Minneapolis School District has locked down all of its schools after police informed officials of vague threats posted on the Web.

District spokeswoman Emily Lowther says the threats were posted on two social networking sites. The threats didn't specify a school, so they were all put on alert.

She says the 34,000 students in the district continue to go to class on Wednesday, but access to the buildings is restricted.

The lockdown started Wednesday morning and Lowther says it's not clear how long it will last.

Lowther says the district has notified parents with its automated telephone system. Another announcement will go out when the district knows more.

She says the Minneapolis Police Department is investigating.