A drone aircraft being used to take air samples at Japan's crippled Fukushima Daiichi nuclear power plant crash landed on the roof of the Unit 2 reactor.

A spokesman for the plant operator, Tokyo Electric Power Company, said Friday that the drone's engine malfunctioned and that the cause was under investigation.

The U.S. made drone was being used to measure radiation levels in parts of the plant that are hard to reach. The spokesman said the drone did not damage the roof of Unit 2 reactor — the only reactor to still have a roof following the March 11 earthquake and tsunami.

The Associated Press said a second machine being used to help measure radiation levels at the Unit 2 reactor also failed Friday. It said a specially-made robot got stuck on a staircase landing.

The Quince robot had been trying to measure radiation levels in the water pooling in the basement. Radiation levels in that area have been too high, making it unsafe for humans to do the testing.

The technical setbacks come just one day after TEPCO announced a system to decontaminate the radioactive water in the plant's reactor buildings was not working as expected.

The recently installed U.S.-built device has reduced radioactive cesium in the water to one percent of the previous level. But that is still 10 times higher than it was supposed to have been.

The system is critical to efforts to bring the plant under control as operators are running out of places to store the highly radioactive water after it is used to keep the plant's nuclear fuel from overheating. The recycling system will allow them to use the same water over and over, removing the risk of the overflow spilling into the sea.