Tokyo's utility company discovered Monday what it suspects could be nuclear fuel debris inside of a reactor at its destroyed Fukushima plant in Japan.

The Tokyo Electric Power Company (Tepco) has led efforts to clean up the Fukushima Daiichi Nuclear Power Plant after three of its reactors melted down in 2011 following a massive magnitude 9.1 earthquake and tsunami that killed over 15,000 people and caused the world's worst nuclear disaster since Ukraine's Chernobyl explosion in 1986. The company discovered black lumps resembling a substance that had melted and stuck to the steel of the No. 2 reactor.

"This is a big step forward as we have got some precious data for the decommissioning process, including removing the fuel debris," said an official quoted by Reuters. Yuichi Okamura, the general manager of Tepco’s nuclear power and plant siting division, said the anomalies were still "difficult to identify," according to The Japan Times.

The 2011 disaster forced about 160,000 residents to flee their homes in the area and persisting radiation levels have left a 310-square mile exclusion zone. Much of the contamination stems from the cooling water dumped onto the reactor's cores. The water leaked through the plant and mixed with groundwater, which the government has successfully diverted from the site. In order to complete the $188 billion recovery process, Tepco must collect the fuel debris remaining in the reactors within the site itself.

Monday's discovery was made by a remote-controlled camera fixed to the lower part of the No. 2 reactor's containment vessel, according to Japanese daily Asahi Shimbun's English-language site. The company developed a robot called "Sasori" (Japanese for "Scorpion") scheduled to investigate the facility's underwater tunnels in search of more nuclear fuel debris.