TOKYO, AP -Thousands of residents were evacuated in Tokyo on Sunday while authorities dug up an unexploded 550-pound bomb, believed to have been dropped by the United States during World War II, a local official said.

The bomb, about 14 inches in diameter and 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 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 six inches by 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 U.S. 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.