Washington, D.C ? Transportation Secretary Norman Mineta, who helped rebuild confidence in U.S. airports and flying after the Sept. 11, 2001, attacks, said Friday he’s leaving the Bush administration.

“It is time for me to move on to other challenges,” Mineta said in a letter to President Bush.

White House press secretary Tony Snow announced the July 7 resignation. When asked why Mineta, 74, decided to leave, Snow said, “Because he wanted to.”

Bush said Mineta – the only Democrat in his Cabinet – played a vital leadership role in strengthening the security of seaports and airports.