Thousands of residents were evacuated in Tokyo on Sunday while authorities dug up an unexploded 250-kg (550-pound) bomb, believed to have been dropped by the United States during World War II, a local official said. The bomb, about 36 centimeters (14 inches) in diameter and 120 centimeters (47 inches) long, was detected earlier this month in a residential area in Tokyo's Katsushika ward by Self-Defense Force investigators, said Katsushika spokesman Takanori Kato. About 3,900 residents within a 300-meter (985-feet) radius of the site were evacuated for 1.5 hours while troops removed the bomb, according to Kato. The evacuation came after a diver found 59 unexploded shells in waters near Tokyo on Friday, believed to have been left by Japan's former Imperial Army. The shells, about 15 centimeters (six inches) by 50 centimeters (20 inches), were found near a port just west of Tokyo during dredging work. It was not clear when the shells were placed there. Unexploded bombs from US attacks or arms hidden by the Japanese during the closing days of World War II in 1945 frequently turn up in Japan. Troops defuse the bombs and often dispose of them at sea.