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Who Bought A Large Number Of Shares?
SEC forms are required when declaring insider activity.  An insider is defined by the SEC to be a person or entity which (i) beneficially owns 10% or more of the outstanding shares of the company, (ii) is an officer or director of the company, or (iii), in the case of insider trading, does so based on knowledge which is not otherwise publically available at the time.  At any rate, the person or entity trading the stock is required to file certain forms. Form 3 is filed when a person first transitions into the status of an insider (by becoming an officer, director, or beneficial owner of a certain percentage of stock). Form 4 is filed when an existing insider trades stock under the company's symbol. Form 5 is filed when certain insider trades of small value are reported later than usual. *More information can be found at the SEC's website. Another possibility is that a large number of options or derivatives were exercised by an officer, director, or lending institution.  In the cases of officers or directors, this would need to be declared with an SEC form 4.  For an institution exercising warrants obtained as a result of a lending agreement, either form 3 or 4 would need to be filed. In addition to the above possibilities, username passing through pointed out a very likely scenario in his answer, as well.