While the discovery of 195 becquerels of strontium-90 in the rooftop sediment fueled concerns that leaked radiation may have spread farther than the central government expected, the officials said the city is carefully examining where the isotope came from.

If the substance is from the Fukushima plant, it will be the first time strontium at a concentration of more than 100 becquerels per kilogram has been found beyond 100 km from the troubled plant.

The strontium-90 was detected by a private agency that conducted the test at the request of a resident.

“Radioactive substances tend to accumulate in sediment and so we still don’t know whether the substance found in this test came from the nuclear accident,” said an official of the Ministry of Education, Culture, Sports, Science and Technology.

Strontium-90, with a half-life of 29 years, had been detected at concentrations roughly between 10 and 20 becquerels at various places across Japan prior to the nuclear crisis.

After learning about the findings, the Yokohama Municipal Government started investigating soil samples collected from areas near the building, the officials said. Meanwhile, the science ministry said it is still uncertain whether the strontium came from Fukushima No. 1.

The ministry has detected radioactive strontium at various locations in Fukushima and Miyagi prefectures within 100 km from the crippled plant in earlier investigations following the accident.

If inhaled or ingested, strontium tends to accumulate in bones just like calcium. It is believed to cause bone cancer and leukemia.