ISTANBUL - Police were on high alert across Turkey on Sunday after the previous day's attack by a suicide bomber in Istanbul's most popular shopping district, killing three Israelis and an Iranian, with dozens more people wounded in the blast.President Benjamin Netanyahu told reporters on Saturday that Israel was trying to determine whether the attack had been aimed at Israelis and was checking if Islamic State was responsible."There is information that it is an attack carried out by an ISIS member," he told reporters on Saturday. "But this is preliminary information; we are still checking it."Israel has confirmed that three of its citizens had died in the blast. Two of them held dual citizenship with the United States. An Iranian was also killed, Turkish officials said.