Relatives of victims of the Sept. 11 terrorist attacks are threatening to hold their own anniversary ceremony unless city and state officials reverse their decision to move the event from the World Trade Center site. "As America watches the memorial service on Sept. 11, 2007, we are sure that they will agree that this event should take place at the real and sacred location," 11 leaders of Sept. 11 family groups wrote in a letter released Saturday. They also questioned how thousands of mourners would be able to fit into a park southeast of ground zero. The park covers about a quarter of an acre (0.3 hectare). Gov. Eliot Spitzer and Mayor Michael Bloomberg said in a letter recently to victims' families that the ceremony would take place at Zuccotti Park because of the expanding rebuilding work at the trade center site. The letter sent to the mayor's and governor's offices proposes the use of a smaller space including a construction ramp and haul road for the ceremony. The letter gives a deadline of Tuesday for city and state officials to respond. After that, the groups say they will apply for a permit to hold their own ceremony at ground zero.