WASHINGTON, Sept. 25 (UPI) -- The U.S. Department of Homeland Security announced plans to conduct the largest and most comprehensive counter-terrorism exercise to date.

The exercise in Arizona, Oregon and the U.S. territory of Guam from Oct. 15-19 is the fourth in a series of congressionally mandated exercises. Top Officials 4 say the exercise will involve 15,000 participants from all levels of government as well as the governments of Australia, Canada and the United Kingdom. TOPOFF 4 will also include the private sector in a full-scale, simulated response to radiological dispersal device attacks.

“The National Intelligence Estimate and recent activity overseas reinforce that we are in a period of increased risk,” said Homeland Security Secretary Michael Chertoff, in a statement. “Exercises like TOPOFF help test response capabilities at all levels, strengthen national preparedness and deepen international coordination. By responding realistically to these simulated attacks, we’re able to identify our strengths and weaknesses, build better partnerships, and gain valuable knowledge for securing the nation against terrorist attacks and other natural disasters.”

Officials say TOPOFF 4 will focus on five areas in an effort to test interagency coordination, planning and preparedness including prevention, intelligence and investigation, incident management, public information and evaluation.

“TOPOFF stresses our preparedness and response systems with situations that no single agency or jurisdiction could handle on its own,” said David Paulison, Federal Emergency Management Agency administrator. “The right response actually takes thousands of individuals working together. This exercise is about strengthening working relationships within our partners in federal, state and local agencies, emergency management communities and private industry groups. It’s about increasing preparedness by sharing information and processes.”