Steam or vapors appear to be coming from a damaged reactor building at Japan's tsunami-crippled nuclear plant, but the plant operator Tokyo Electric Power Co (TEPCO) has said radiation levels are steady.
The steam rising from the reactor number three building was spotted on Wednesday at 8:20am (23:20 GMT) by a subcontractor who was filming the destroyed building and preparing to remove rubble from the site.
TEPCO on Thursday said that the reactor's spent fuel pool is stable and measurements of the temperatures and pressure have not changed significantly.
Workers were continuing to inject water into the number three reactor to cool it, the utility said.
"We think it's possible that rain made its way through the reactor building and having fallen on the primary containment vessel, which is hot, evaporated creating steam," said TEPCO spokeswoman Maymi Yoshida, adding it was still investigating the matter.
The reactor was one of three at the Fukushima Dai-ichi plant that suffered core meltdowns after the 2011 earthquake and tsunami, leading to the world's worst nuclear disaster since Chernobyl in 1986.
Thousands of people have been unable to return to their homes near the plant because radiation levels are still high.
Most of Japan's nuclear reactors remain shut down for safety checks following the disaster.
Hard to cool down
The latest findings underscore the difficulties TEPCO is facing in trying to keep the ravaged plant under control.
About a week ago, a huge spike in radioactive cesium was detected in groundwater 25m from the sea.
The operator has been flushing water over the damaged reactors to keep them cool for more than two years, but contaminated water has been building up at the rate of an Olympic-size swimming pool per week.
In April, TEPCO warned it may run out of space to store the water and asked for approval to channel what it has described as groundwater with low levels of radiation around the plant and to the sea through a "bypass".
Local fishermen oppose the proposal.