Knight shares plunged $3.39, or 33 percent, to $6.94 on Wednesday and in premarket trading Monday, they gave up another 32 percent, falling to $2.76.
Going in the opposite direction is Best Buy Co., which jumped 24 percent in premarket trading after Richard Schulze offered to buy the company.
Schulze, 71, founded the company in 1966 and is its largest shareholder by far with a 20 percent stake.
Schulze hinted two months ago that he might dump his stake in the company and in early June, resigned as chairman.
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