Nikola Motor Company Two truckSource: Nikola Motor CompanyShares of Nikola plummeted by as much as 18.3% in heavy trading Tuesday morning after a lockup period on the stock for company insiders and early investors expired, freeing up to 166 million shares for sale for the first time since the embattled electric truck company went public in early June.A majority of those shares – 91.6 million – are held by Nikola founder Trevor Milton, who resigned as chairman of the company in September. A spokesman for Milton declined to comment Tuesday morning about whether the former executive had sold any shares.More than 50 million Nikola shares had been traded as of 11:20 a.m. Tuesday. That compares with a five-day average of 47.7 million shares and 30-day average of 20.5 million, according to FactSet.Read More: Nikola founder Trevor Milton can sell his 91.6 million shares starting Tuesday as insider lockup period expiresNikola shares were trading at under $17.30 as of 10:59 a.m. The decline adds to an already volatile week for the stock.Shares of Nikola tanked by 26.9% to $20.41 Monday after the company announced a scaled back deal with General Motors before the markets opened. GM gave up an equity stake in the start-up as well as plans to produce the Badger, an electric pickup that was supposed to be the company’s first foray into the consumer market.Roughly 161 million shares of common stock is eligible for sale beginning Tuesday, according to Nikola’s third-quarter SEC filing. An additional 5 million shares may also be eligible for sale depending on the price of the company’s stock. Milton’s ownership includes 6 million shares in “founder options” he could give out to early employees.Milton stepped down after the Department of Justice and Securities and Exchange Commission started investigating allegations of fraud raised by short-seller Hindenburg Research in September.