Updated

The U.S. government's new system to replace the five color-coded terror alerts will have two levels of warnings -- elevated and imminent -- that will be relayed to the public only under certain circumstances for limited periods of time, sometimes using Facebook and Twitter, according to a draft Homeland Security Department plan obtained by The Associated Press.

Some terror warnings could be withheld from the public entirely if announcing a threat would risk exposing an intelligence operation or a current investigation, according to the government's confidential plan.

Like a carton of milk, the new terror warnings will each come with a stamped expiration date.

The 19-page document, marked "for official use only" and dated April 1, describes the step-by-step process that would occur behind the scenes when the government believes terrorists might be threatening Americans. It describes the sequence of notifying members of Congress, then counterterrorism officials in states and cities and then governors and mayors and, ultimately, the public. It specifies even details about how many minutes U.S. officials can wait before organizing urgent conference calls among themselves to discuss pending threats. It places the Homeland Security secretary, currently Janet Napolitano, in charge of the so-called National Terrorism Advisory System.

The new terror alerts would also be published online using Facebook and Twitter "when appropriate," the plan said, but only after federal, state and local government leaders have already been notified. The new system is expected to be in place by April 27.

The government has always struggled with how much information it can share with the public about specific threats, sometimes over fears it would reveal classified intelligence or law enforcement efforts to disrupt an unfolding plot. But the color warnings that became one of the government's most visible anti-terrorism programs since the September 2001 attacks were criticized as too vague to be useful and became fodder for television comedians.

The new advisory system is designed to be easier to understand and more specific, but it is impossible to know how often the public will receive these warnings. The message will always depend on the threat and the intelligence behind it.

For example, if there is a specific threat that terrorists were looking to hide explosives in backpacks around U.S. airports, the government might issue a public warning that would be announced in airports telling travelers to remain vigilant and report any unattended backpacks or other suspicious activity to authorities.

If the intelligence community believes a terror threat is so serious that an alert should be issued, the warning would offer specific information for specific audiences. The Homeland Security secretary would make the final decision on whether to issue an alert and to whom -- sometimes just to law enforcement and other times to the public.