WASHINGTON — Homeland Security Secretary Janet Napolitano says the color-coded terrorism alert system will be replaced next week by two warnings — elevated and imminent.

The new warnings could be conveyed to the public through Facebook andTwitter. Unlike the colors, the alerts will have expiration dates.

The past system described the threat in five colors, from green, low risk, to red, severe.

Napolitano says the threat picture in the U.S. now is at an “elevated baseline,” and future public warnings will hinge on specific and credible intelligence above and beyond that threshold.

She says the terrorist threat to the U.S. is at its highest since 9-11 because of the increasing number and types of international and homegrown threats. This does not mean there is a specific threat of an imminent attack.

“The Department [of Homeland Security] must ensure that the new system will effectively disseminate threat information in a timely manner and provide sound guidance to the public and affected homeland security partners on the actions they should take to protect themselves and the nation. I urge DHS to keep communication lines open with our state and local homeland security officials and with the private sector so that implementation of the system meets the vision set by the Secretary,” said Sen. Susan Collins, R-Maine, who is ranking member of the Senate Homeland Security Committee.