In a threat still considered uncorroborated, intelligence sources overseas informed the FBI that hospitals in four U.S. cities could be the targets of terrorists, Fox News has confirmed.
But the White House said Thursday that the credibility of the threats was "very low."
Houston FBI spokesman Bob Doguim said Wednesday night the vague threat involved hospitals in Houston, San Francisco, Chicago and Washington, D.C., and mentioned a time between December and April.
"It's non-specific, uncorroborated information, but nonetheless it is information we received," Doguim said.
He said the threat mentions the possibility of anthrax or explosives.
"There was word that that could possibly, possibly be the way they carry out the attacks," he said.
"The alleged attack would take place in reaction to the continued arrest of a Pakistani national by Pakistani authorities," according to an FBI statement.
White House deputy press secretary Scott McClellan said Thursday: "The information given to local law enforcement by the FBI regarding a possible threat to hospitals ... has very low credibility. It is uncorroborated and that is why it was not made public."
A statement from the Chicago FBI office said the threat suggested an attack was timed for mid-December and the holidays.
"We've always been on a state of high alert," Chicago officer Carlos Herrera said. "That hasn't changed."
Officials do not know a specific date when the threats could be carried out, FBI special agent Bill Farhat said.
FBI officials informed the Chicago Police Department of the potential threat, Herrera said.
Chicago police officials have notified officers of the threat and told them to be "on the lookout for anything suspicious," Herrera said.
He said police do not know what the specific threat could be, but that they are notifying hospitals of the potential danger.
Hospital officials at Loyola University Medical Center in Maywood were not told about the alert, hospital spokesman Stephen Davidow said.
Even though the hospital has not been notified of any specific threats, Illinois hospitals are required to have a disaster plan, he said.
"We are, of course, looking into further things as part of the aftermath of 9/11," Davidow said. "As a Level 1 trauma center, we deal with disasters of all kinds. We've got a lot of stuff in place."
Fox News' Carl Cameron and The Associated Press contributed to this report.