The Japanese government is investigating the distribution of crushed stones that may contain radioactivity from the accident at the Fukushima Daiichi nuclear power plant. It has found that concrete made of the stones has been sold to more than 200 firms.

The probe comes after radioactive cesium was detected in a new apartment building in Nihonmatsu, Fukushima Prefecture where the concrete was used. Readings of up to 1.24 microsieverts per hour have been recorded inside the building which is higher than outside.

The gravel comes from a quarry in Namie that was designated as an evacuation zone in April.

The quarry operator says it shipped more than 5,000 tons of crushed stones to 19 firms in the weeks after the accident. The government has set no limit on radiation in stones and sand used for construction even after the nuclear accident.

The company president told reporters on Monday that he never imagined that stones from his quarry would cause such a problem and that he wants to apologize to residents of the building for their exposure to radiation.

The economy ministry says 2 of the 19 firms sold concrete made of the stones to more than 200 companies. It believes that the concrete was used in housing construction and on roads.