Frigid cold and rain forced more than 400 undocumented immigrants crossing the reservation in the southern Arizona desert to turn themselves in overnight, authorities said April 6. Most of the 304 illegal immigrants sought assistance from tribal officers near Sells, tribal police Chief Larry Seligman said. About another 100 people turned themselves in to Border Patrol agents, said agency spokesman Rob Daniels. In March 2000, a storm had 350 shivering immigrants knocking on doors and stepping from darkened paths to flag down police and ask for shelter. This time, five people were airlifted or taken by ambulance to Tucson hospitals for treatment of hypothermia, Daniels said. Tribal police set up an emergency center with the Red Cross at a middle school, providing blankets and food for the immigrants. "They would find us, we didn't have to look very hard,'' Seligman said. "They would be on the roadways ... and if they saw you were in a police vehicle they would wave you down.'' The Border Patrol's Tucson sector - all of the Arizona-Mexico border except for the area around Yuma - is the nation's busiest illegal immigration corridor. But an influx of agents and resources has increasingly led smugglers to direct ill-equipped immigrants into remote, rugged desert terrain.