Texas officials said Nevada also received a similar threat.
But an FBI agent in Las Vegas called reports of threats against Nevada schools "totally and completely uncorroborated."
"There's nothing substantiated. There are no specific threats or targets," said Special Agent Gayle Jacobs, spokeswoman for the Las Vegas FBI office. The southern Nevada office also covers northern Nevada.
Jacobs said she was unaware if schools in Nevada had been contacted.
Frank Siracusa, head of Nevada's Emergency Management Division, and Greg Bortolin, Gov. Kenny Guinn's press secretary, said they had heard nothing about a threat to Nevada.
Texas Gov. Rick Perry called the threat "low-level" and said, "I believe Texas is safe and our schools are safe."
"The FBI considers it to be a low-level threat. Nonetheless, I believe it's important that school officials are notified and that parents, teachers, administrators and law enforcement personnel are extra vigilant," Perry said.
"Texans sent their children to school today and they should do so tomorrow," Perry said.
Houston FBI Special Agent Bob Dogium also called the Texas threat "vague" and said the information came from a foreign government.
"We are working with them to try and determine the reliability of it," he said.
A law enforcement official in Washington, speaking only on condition of anonymity, said authorities have not determined if the Texas threat is credible. The Justice Department is monitoring, he said.
Dogium said the FBI informed Texas' Homeland Security Task Force, which began informing public school districts, private schools, colleges and universities late Tuesday night after receiving the information.
"The initial concern is that this might cause undo panic," he said. "Our responsibility is to let them know this is out there."
Perry said he couldn't provide specific details of the threat. Texas Department of Public Safety Director Col. Thomas Davis Jr. said Nevada was also threatened.
Land Commissioner David Dewhurst, chairman of Texas' homeland security task force said the alert means security will increase in and around schools and will remain in effect until the threat is cleared by law enforcement.
The Texas Education Agency did not cancel classes. The agency sent an e-mail to the superintendents of the state's 1,183 school districts Wednesday alerting them of the threat.
"At this point, it may prove to be unfounded but it provides us all with a good opportunity to review our security and emergency procedures," said spokeswoman Debbie Graves-Ratcliffe.
In Houston, Dogium said agents aren't sure how accurate the information is, but said it is not related to a specific school.
Other threats around the nation made public in recnt months never have materialized.
In October, hospitals in Baltimore were warned of a possible anthrax attack. The FBI told police there it had learned the attack might come Oct. 17, the same day first lady Laura Bush was visiting children at an elementary school.
In early November, a threat of rush-hour terror attacks against four California bridges turned out to be not credible, causing California Gov. Gray Davis to face criticism for announcing the threat publicly. The FBI said the information was uncorroborated but Davis said he believed he had an obligation to warn the public.
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