Shares of Tesla Inc. /zigman2/quotes/203558040/composite TSLA +5.04% sank 8.4% in morning trade Wednesday toward a one-year low, extending the previous session's 8.2% tumble, amid growing liquidity concerns and after the National Transportation Safety Board launched an investigation into a fatal crash last week. The stock's two-day plunge of 15.9% puts it on track to suffer the worst two-day performance since it lost 21.0% in the two days ending Nov. 7, 2013. The stock has now plummeted 25.4% so far this month, which would be its worst month since the electric car company went public on June 29, 2010, at an initial public average of $17. In comparison, the S&P 500 /zigman2/quotes/210599714/realtime SPX +1.60% has lost 3.9% this month. Tesla's previous worst month was December 2010, when the stock tumbled 24.6%.