Washington, D.C. ? Fran Townsend, the leading White House-based terrorism adviser who gave public updates on the extent of the threat to U.S. security, is stepping down after 4 1/2 years.

President Bush said in a statement Monday morning that Townsend, 45, “has ably guided the Homeland Security Council. She has played an integral role in the formation of the key strategies and policies my administration has used to combat terror and protect Americans.”

Her departure continues an exodus of key Bush aides and confidants, with his two-term presidency in the final 14 months. Top aide Karl Rove, along with press secretary Tony Snow, Attorney General Alberto Gonzales, Defense Secretary Donald Rumsfeld and senior presidential adviser Dan Bartlett, have already left.