Some epidemiologist studies traced the cause to consumption of L-tryptophan from a single Japanese manufacturer, Showa Denko. The company supplied the majority of L-tryptophan to the United States under various brand names. There was evidence that new batches of L-tryptophan may have been improperly prepared. First, the specific bacterial culture used to synthesise this tryptophan had recently been genetically engineered to greatly increase tryptophan production. The increased concentrations of tryptophan in the fermentor may in turn have led to increased production of trace impurities. Second, shortcuts had been taken in the purification process to reduce costs. For example, a purification step that used charcoal absorption to remove impurities had been modified to reduce the amount of charcoal used. It is possible that one or more of these modifications and/or the environment for manufacture allowed new or greater impurities through the purification system. More than 60 different impurities were identified in the L-tryptophan lots which had been associated with cases of EMS.
The specific impurity (or impurities) responsible for the toxic effects was never firmly established, however two compounds, EBT (1,1'-ethylidene-bis-L-tryptophan) and MTCA (1-methyl-1,2,3,4-tetrahydro-beta-carboline-3-carboxylic acid), which are close chemical relatives of L-tryptophan were implicated.
Regardless of the origin of the toxicity, L-tryptophan was banned from sale in the US in 1991; and other countries followed suit. In February 2001, the FDA loosened the restrictions on the marketing of tryptophan (though not on importation).