NEW YORK (CNN/Money) - Shares of Merck & Co. fell 4 percent Wednesday after the maker of the cholesterol drug Zocor and arthritis drug Vioxx repeated a disclosure that the Justice Department is investigating how Merck sold and marketed its drugs.

The information, in a Securities and Exchange Commission filing dated Wednesday, followed a nearly identical disclosure in a March 21st filing.

"There's nothing new," said Merck spokesperson Janet Skidmore, who could not provide details about the investigation.

But investors appeared spooked by the probe, which was reported by at least three other news organizations.

Merck (MRK: Research, Estimates) shares fell $2.20, or 4 percent, to $52.80 Wednesday during the regular trading session; more than 12 million shares changed hands, twice the normal daily volume. But the company's stock bounced back slightly in after-hours trading Wednesday, rising 20 cents to $53, according to Instinet.

By comparison, Merck shares fell only 3 cents amid normal trading volume of 6.6 million shares when the probe was first detailed on March 21.

"Merck has been advised by the U.S. Department of Justice that it is investigating marketing and selling activities of Merck and other pharmaceutical manufacturers," the company's 10-Q filing stated Wednesday. "Merck will be working with the government to respond appropriately to informational requests."

A Justice Department spokesman said he had no details about the probe. The Merck spokesperson could not say what other drug companies were involved.

Separately, the West Virginia Attorney General Wednesday said Wednesday that it sued Merck's pharmacy benefits unit, Merck-Medco, claiming that the unit cost state employees "millions" instead of saving them money as promised. The complaint was filed in the Circuit Court of Kanawha County, West Virginia.

Other drug stocks also fell Wednesday, including Pfizer (PFE: Research, Estimates), which lost 84 cents to $32.60, and Wyeth (WYE: Research, Estimates), which declined 60 cents to $34.40.Eli Lilly (LLY: Research, Estimates) shed $1.10 to $60.29.

The DOJ inquiry is one of many drug-company probes. Schering-Plough Corp. (SGP: Research, Estimates) Tuesday said it received two new grand jury subpoenas from federal investigators in Massachusetts.

Last month, Bristol-Myers Squibb (BMY: Research, Estimates) said that the U.S. Attorney's office in New Jersey is looking into the company's wholesale inventories, joining federal regulators who want to know if the company inflated sales on purpose by offering incentives to wholesalers.