SAN FRANCISCO  (AP) — Twitter’s stock sank to an all-time low after a post-IPO lock-up period preventing employees and early investors from selling expired on Tuesday.

Lock-up periods prevent company insiders from selling stock following an initial public offering. CEO Dick Costolo and co-founders Jack Dorsey and Evan Williams have said that they had no plans to sell their stock when the lock-up expired, 180 days after Twitter’s initial public offering.

Still, Twitter’s stock plunged nearly 18 percent to close at $31.85 on Tuesday. Earlier, its shares hit their lowest point ever at $31.72. Trading volume was unusually heavy.

San Francisco-based Twitter Inc. went public on Nov. 7. The stock soared as high as $74.73 in December but has declined sharply since. The company’s latest earnings report on April 29 surpassed expectations, but worries about user growth and engagement weighed have on its stock.