Researchers from the United States and Europe say Japan's crippled Fukushima nuclear plant may have released twice as much radioactive material as first thought.

A study posted Thursday on the website for the journal Atmospheric Chemistry and Physics says the plant likely spewed twice as much cesium-137 into the atmosphere than initially reported. The conclusion was based on an analysis of sensors from around the world.

Lead author Andreas Stohl, with the Norwegian Institute for Air Research, says the reason for the difference is that Japanese officials were not able to measure radioactive material that was blown out to sea.

In all, the amount of cesium-137 released into the atmosphere reached about 40 percent of that released during the meltdown at Chernobyl .

According to the U.S. Environmental Protection Agency, exposure to cesium-137 can cause an increased risk of cancer.

No one has yet died from radiation exposure, but tens of thousands of people remain evacuated from areas in and around a 20-kilometer zone around the the disabled Fukushima-Daiichi plant.

The power plant has been leaking radiation since the March 11 earthquake and tsunami devastated much of coastal northeastern Japan, leaving 20,000 people dead or missing.