Tokos Confirms Investigation by FDA : Stocks: A ‘short seller’ reportedly complained to authorities, a company official says. However, the nature of the complaint remains unknown.

Tokos Medical Corp. confirmed Friday that it is under investigation by the U.S. Food and Drug Administration.

Tokos President Craig T. Davenport said neither he nor other company officials know the nature of the probe by federal health authorities.

“We were told that they had received a complaint that was sent to them by a ‘short seller,’ ” Davenport said late Friday afternoon.

Short sellers are investors who make money when a company’s stock loses value. That can happen if investors sell off shares because negative reports about a company make them jittery. Davenport said that about 5 million of the company’s outstanding shares are now controlled by short sellers.

The news Friday caused the stock’s price to fall, although not substantially. In trading on the NASDAQ market, it closed at $15.75 a share, down 12.5 cents.

Davenport said he knew about the FDA investigation in “late August and early September.” The agency has not given details of the probe, he said, other than to tell Tokos officials that a professional short seller had lodged a complaint.

The FDA office in Los Angeles would neither confirm nor deny that any Tokos investigation is ongoing. But a spokeswoman in the FDA’s Washington office told Bloomberg Business News on Friday that the agency is conducting an “active and ongoing” investigation into the company’s practices.

That revelation marked the second time this week that Tokos has acknowledged that an inquiry was underway. The company, the largest of its kind in the United States, manufactures a device that can detect the onset of early childbirth and provides nurses to monitor the device. That device has been criticized repeatedly by some physician groups that question its viability. So far, the company has successfully weathered such criticisms as well as a series of ups and downs on Wall Street.

But the latest criticisms of the company have not focused on the controversial device. The American Medical Assn. said Tuesday that it is reviewing a complaint that Tokos may be engaged in unethical practices because of a “reimbursement schedule” it sent to about 100 doctors in Utah and Colorado.

The review by the AMA’s Council on Ethics and Judicial Affairs during the association’s semiannual gathering in Nashville, Tenn., is expected to be completed by Sunday, an AMA lawyer said.