WASHINGTON (AP) — The nation’s top domestic security official said Wednesday aviation remains vulnerable to terrorist attack seven years after 9/11. Homeland Security Secretary Michael Chertoff said the al-Qaida terrorist network continues to focus on the aviation system as a target. He said that the Bush administration has made strides in reducing the nation’s vulnerability but that the risk remains. Chertoff was speaking at the National Press Club. Thursday is the seventh anniversary of the Sept. 11, 2001 attacks, in which terrorists crashed hijacked airplanes into the World Trade Center, the Pentagon and a Pennsylvania field. White House spokeswoman Dana Perino said the president thinks about 9/11 every day and is always concerned about another attack.