Students in 12 northern New Jersey towns got an unexpected day off Thursday after a threatening letter warned that several schools would be "blown out" at 11:30 a.m.

Emerson's mayor received the letter Wednesday, and police and sheriff's deputies searched about 60 school buildings there and in surrounding northern New Jersey towns through the night but found no explosives, Bergen County Police Capt. Kevin Hartnett said.

Classes, canceled for about 14,000 students Thursday, were to resume Friday.

"It's important that we respond appropriately to any threat to our schools or students," Emerson Police Chief Michael Saudino said.

The letter appeared to be computer-generated and was being examined for clues. It threaten Emerson's three schools and two others in unnamed nearby towns.

Officials said they chose to err on the side of caution in canceling class and searching all the schools. Police and sheriff's departments from New Jersey and New York lent officers and bomb-sniffing dogs.

In addition to Emerson, the threat closed schools in Oradell, River Edge, Closter, Demarest, Haworth, Harrington Park, Northvale, Norwood, River Vale, and Old Tappan, along with Northern Valley and River Dell regional high schools, and the Westwood Regional district. Administrators at several nearby private schools also called off classes.