HOUSTON - A US nuclear laboratory may have allowed materials incompatible with nuclear waste to be used in containers believed to be the source of a major radiation leak at a storage site, according to internal documents released Thursday.

Investigators suspect a change in the kind of kitty litter, used to absorb moisture in nuclear waste, triggered a chemical reaction in at least one container stored at the Waste Isolation Pilot Plant (WIPP), an underground nuclear waste dump in southeastern New Mexico, which led to a radiation leak that has contaminated 22 workers.

The suspect containers were shipped from Los Alamos National Laboratory, a leading nuclear weapons manufacturer located in New Mexico.

New Mexico Environment Department on Thursday made public online internal Los Alamos emails showing the lab approved products which contain ingredients widely known to cause a heat reaction when combined with the containers' other contents.

The emails traced the laboratory's approval of products requested by contractor EnergySolutions, which packages the waste, to neutralize the pH balance of container contents sent to WIPP, local media reported.

EnergySolutions asked Los Alamos managers for approval in a May 2013 email to switch to one of the products. The Laboratory approved the change a month later -- despite product warnings clearly stating the product is incompatible with metallic nitrates and "strong oxidizers."

Nitrates and organic matter are known to oxidize, a reaction that generates heat.

The emails didn't say what kind of testing may or may not have been done at Los Alamos before the products were approved. Experts say it's common sense that a new product must be thoroughly tested before being put into use.

Though the kitty litter theory remains the most viable, officials did not exclude other possibilities. Both Los Alamos and WIPP have reacted cautiously to the theory and made no conclusions.

More than 500 waste containers in question are stored at the Los Alamos campus, a temporary storage site in Texas and at WIPP, most of them at the latter site. Officials are urging all three facilities to keep a close eye on the potentially hazardous containers. WIPP will reportedly seal off the underground vaults where the containers are held.

The nuclear leak happened on Feb.14 when sensors detected unusually high levels of radioactive particles at WIPP. It has been shuttered since.

The WIPP stores "transuranic waste" left over from nuclear weapons for 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.