WASHINGTON (CNN) -- A veil of secrecy will begin to rise Wednesday when a congressional committee reveals some of the information that U.S. spy agencies knew about suspected terrorist activity before the September 11 attacks -- including the disclosure that analysts knew al Qaeda had previously plotted to use aircraft as terrorist weapons. The recently declassified documents will be made public for the first time during an open hearing of the joint House-Senate Intelligence Committee. One of the main questions investigators are probing: Could the September 11 attacks have been prevented? Joint Senate House Intelligence hearings CNN plans coverage of...