In this 2001 UK Guardian article about a covered-up mass pesticide poisoning in Spain, the journalist asserts that the same thing was behind the 1989 deaths in the US attributed to tainted L-tryptophan supplements. The reference in question is about 5/6 down the page, in a short paragraph. The article still reflects the government ban on tryptophan that lasted a few years.
FYI I have not looked into this topic beyond a brief internet search, which revealed a lot of questioning of the official story, but with suspicions falling on the genetic-engineering aspect of the supplements’ manufacture.