The Food and Drug Administration said yesterday it was continuing its investigation of quality control issues at Mylan Inc.'s generic drug making plant in Morgantown, W.Va., contrary to a company statement earlier in the day that said the probe was over and had found only "minor" problems.

"This investigation involves allegations of compliance violations that the FDA takes very seriously," FDA assistant commissioner Steven Solomon said in a statement. "The investigation is ongoing and the agency has formed no conclusions at this time. Statements to the contrary are untrue."

The agency's statement came two hours after Mylan issued a news release saying the FDA had visited the Morgantown plant Monday morning and had found "a minor standard operating procedures deviation" and that "all corrective actions were fully implemented."

Mylan yesterday did not respond to a request for comment about the discrepancy. However the company told other media that it believed the confusion stemmed from a delay in the local FDA official informing headquarters about the inspection.

But the agency said there was no confusion, reiterating that the investigation was ongoing.

"We stand by our earlier comment," spokesman Christopher Kelly said late yesterday.

Shares of Mylan see-sawed yesterday as investors digested the competing statements from Mylan and the FDA. Shares reached as high as $13.37 and as low as $11.97 before closing at $12.56, up 46 cents, or 4 percent. Some 46 million shares changed hands, about six times normal trading volume. Shares slid 13 percent on Monday, when more than 50 million shares changed hands.

A story published Sunday by the Pittsburgh Post-Gazette cited a Mylan internal report that stated workers routinely overrode computer-generated warnings about potential problems with the medications they were making. The report did not say what drugs were involved.

Mylan's report said the problem was "pervasive" and occurred on all three shifts at the plant, which makes an estimated 19 billion doses annually. It did not say how long the unauthorized practice had been going on.

The breach in quality control was "very serious," involving "falsifying information" and "altering product" but did not affect the quality of the medications, the report stated. The report did not say how the company determined that quality was not compromised.

In its statement yesterday morning, Mylan said the FDA had "determined that the baseless accusations in the article were unfounded."

CEO Robert Coury said in the statement that the FDA's actions "only reaffirm Mylan's long-standing reputation as one of the industry's highest quality manufacturers with a management team that has an unyielding commitment to adhere to the highest quality standards."

Analysts said Mylan's statement yesterday that the FDA had cleared the plant could hurt the company's credibility in the investment community.

"At a time when they [Mylan] had been overcoming all of the previous credibility issues Wall Street had been obsessed about, management in one day undoes all of the goodwill that it has created over the past six to nine months," said Corey Davis, who follows Mylan for Natixis Bleichroeder in New York. Mr. Davis maintains a "buy" rating on the stock.

Mr. Coury's credibility was questioned three years ago when he issued a statement predicting that the company would not have any more competition during fiscal 2007 for its fentanyl pain patch.

That same day, two hours before Mr. Coury's statement, a competitor announced it had received FDA approval for a second generic fentanyl patch.

Last year, Mr. Coury also drew criticism from analysts for not being open or clear enough when discussing the company's financial results.

"Some investors feel that Mylan hasn't shot straight with them in the past," said David W. Maris at Phalanx Investment Partners in New Jersey and a former Mylan analyst at Bank of America. "This whole episode is not going to help them in that regard."