The source of high radiation levels discovered by citizens at as many as 20 spots throughout Tokyo City has been identified as Radium-226, which was kept in bottles in a house, officials said.
The officials also ruled out the possibility of contamination having spread from the crippled Fukushima nuclear plant.
The Head of the Science Ministry's office for radiation regulation, Takao Nakaya, told Bloomberg that an investigation of a house near a contaminated area in Setagaya ward turned up two boxes of bottles containing Radium-226.
The high readings have nothing to do with Fukushima, said Nakaya.
National Broadcaster NHK, on Friday, reported that a citizens' group in Funabashi City in Chiba had detected radiation levels of up to 5.82 microsieverts per hour at a local park, compared to official readings of 1.55 microsieverts per hour at the site.
Other areas in Tokyo and Yokohama have reported radiation readings requiring further investigation, with most readings recorded by local residents using personal dosimeters.