Officials don't know how credible the threat was. But given its specificity and the fact that it extended to more than one school, school district Superintendent Mark Conrad asked all students to stay home.
The Nashua School District includes 12 elementary schools, three middle schools and two high schools.
"We have received a detailed threat of violence to harm students and staff at both high schools," Conrad said Sunday
"Because the threat is specific and extends to several schools, we will be cautious and close all of our public schools in Nashua tomorrow, Monday, December 21st."
Classes will resume Tuesday.
On Monday, Conrad stressed again that the threat was specific. It named the two high schools, cited the means by which students were to be harmed, and was specific as to the date of attack, he said.
"This is not a typical threat. It was very specific, and we have to take that seriously," said Conrad.
Nashua police said a school administrator received an email with the threat. The department would not go into details.
Nashua Police Chief Andrew Lavoie said the cancellation gave law enforcement the time to conduct extensive safety checks at all the schools.
"We can say with confidence that there is no current credible threat to any Nashua public school," he said.
The chief declined to comment on any potential suspects, but said if the threat was a hoax -- and he didn't say that it was -- the perpetrator would be prosecuted to the full extent of the law.
"It's definitely a tightrope," said Nashua police Lt. Kerry Baxter. "It's a tough decision to make. These seem to be going on throughout the country. They seem to be happening more frequently, and they are disruptive on many different levels."
Los Angeles called off school for a day. New York dismissed its threat as a "hoax" and held school.
The threat received by Nashua was not similar to the threats sent to New York and Los Angeles, according to a law enforcement official briefed on the investigation.
The person behind the Nashua threat is likely local, the official told CNN. The person demanded some sort of payment from authorities to prevent an attack, the official said.