Which level of the homeland security advisory system indicates "a significant threat of terrorist attacks"?

"Homeland Security Advisory System" is the most popular security mechanism of the Department of Homeland Security (DHS).

It is 5-level, color coded security system that has been developed to warn the American public about the levels of terrorist threats that has been detected by the intelligence community.

Level 3 or "Yellow" code indicates that there might be "a significant threat of terrorist attacks".

When this code is given, the government agencies increase the surveillance of the important locations, coordinate emergency planning methods, and implement plans for contingency and emergency response.