Days after social media giant Facebook started publicly selling its stock, financial regulators in the U.S. are taking a closer look at whether key information about the company's financial prospects was provided to large investors in advance of the sale, but withheld from others.

In the days leading up to the start of trading in the stock last Friday, investigators say that an analyst at Facebook's leading financial supporter, the Morgan Stanley investment banking firm, may have shared negative information about Facebook's future earnings prospects with large institutional investors. Morgan Stanley said it did nothing wrong.

But analysts say the information was not widely known in advance of one of the largest initial stock offerings in U.S. financial market history. Several investors have sued Facebook for not disclosing information.

The firm's stock opened at $38 a share, and only advanced by 23 cents by the end of Friday trading, a very small gain considering the widespread interest in the stock. Facebook shares plunged Monday and Tuesday, falling more than 19 percent to $31, although it has moved up more than 3 percent in early Wednesday trading.

In addition to questions about the pricing of the stock by Facebook and Morgan Stanley, the NASDAQ stock exchange says it mishandled trading in the stock.

NASDAQ said its technology could not cope with the huge volume of stock trades on the first day and might pay damages to investors who lost money because of the problem. The stock exchange says that had it known of the technology shortcoming in advance, it would have delayed the opening in Facebook stock trading.