HOUSTON, May 13 -- A scientist who worked for a nuclear waste plant in the U.S. state of New Mexico said that "kitty litter" may have been to blame for the radiation release at the facility in February, U.S. media reported Tuesday.

The leak at the Waste Isolation Pilot Plant, near Carlsbad in southeastern New Mexico, exposed 21 workers.

Kitty litter was used at the underground plant to absorb liquid in the toxic waste. Jim Conca, who worked with the facility from 2000 to 2010, told local media the Carlsbad Current-Argus that he believed a change from non-organic kitty litter to organic litter caused a chemical reaction inside a waste drum, releasing the radioactive isotopes.

Officials said previously the radiation leak was likely caused by nuclear waste containing nitrate salt, which resulted in some sort of chemical reaction inside the drums from Los Alamos National Laboratory, a leading nuclear weapons manufacturer in the country. Shipment of that particular waste has been halted in the laboratory in light of the investigation.

The leak occurred on Feb. 14 when sensors detected unusual high levels of radioactive particles in the underground dump. It has been kept shuttered since then and officials said last week it could be up to three years before full operations resume.

The U.S. Department of Energy said in a statement released on Tuesday that it is investigating "all possible scenarios."

The Waste Isolation Pilot Plant stores "transuranic waste" leftover from nuclear weapons research and testing from the nation's past defense activities, according to the Energy Department's website. The waste includes clothing, tools, rags and other debris contaminated with radioactive elements, largely plutonium.