Tokyo Electric Power Company says last week's leakage of untreated water from a filtering device at the damaged Fukushima Daiichi power plant occurred after workers failed to notice that some valves had mistakenly been left open.

The company, TEPCO, revealed the information at a news conference on Thursday.

The finding is based on the company's ongoing investigation into the cause of the leakage, which occurred on February 7. TEPCO has been questioning workers who were at the site when the accident occurred.

The company told reporters that workers had failed to notice that of the 16 valves on the device's pipes that needed to be closed, 10 were left open.

The utility admitted that the operation manual lacked clear instructions for workers regarding the closure of the valves. It promised to take steps to prevent a recurrence.

TEPCO also significantly revised downward the amounts of leaked water and radioactive substances. The firm now says about 1.5 tons of water containing about 6.6 billion becquerels of radioactive substances leaked.

It previously said that about 5.5 tons of water with 22 billion becquerels of cesium-137 and other radioactive substances had leaked.

The company announced on Monday that it had finished removing all of the soil that the leaked water had seeped into.

However, upon examining the soil beneath the concrete-paved ground, TEPCO says it detected radiation levels of up to 7 millisieverts per hour -- significantly higher than pre-leakage level of 0.02 millisieverts.

As a result, the firm said it would also remove this additional soil.