School buses are rolling on Wednesday morning and parents are dropping off children at campuses in the nation’s second-largest school district.

The decision on Tuesday to close the district with 640 000 students came after a school board member received an e-mail threatening a large-scale attack.

District Superintendent Ramon Cortines says he ordered the closure as a precaution.

New York City schools received a similar threat, but officials there concluded that it was a hoax