The mayor of New Orleans has set up an "extremely problematic" timeline for allowing residents to return to the evacuated city, which is still threatened by a weakened levee system, a lack of drinkable water and heavily polluted floodwaters, the head of the federal relief effort said Saturday. Coast Guard Vice Adm. Thad Allen said federal officials have worked with Mayor Ray Nagin and support his vision for repopulating the city, but he called Nagin's idea to return up to 180,000 people to New Orleans in the next week both "extremely ambitious" and "extremely problematic."