UPDATE 2-US FDA clears key Mylan plant, closes probe

* FDA says no additional action planned

* Mylan shares rise 3.9 percent (Adds report details, background, analyst comment, byline)

By Lewis Krauskopf

NEW YORK, Aug 13 (Reuters) - The U.S. Food and Drug Administration said on Thursday it completed a probe into a key Mylan Inc (MYL.O) manufacturing plant and found no deficiencies, clearing an issue that has hung over the world's No. 3 generic drugmaker. Its shares rose about 4 percent.

The FDA said it issued the report to Mylan, which had been under a cloud since a newspaper article raised questions about its Morgantown, West Virginia, plant late last month.

"This officially closes the FDA's investigation and the agency plans no additional action," FDA spokeswoman Judy Leon said.

Since the Pittsburgh Post-Gazette published the article late last month, Mylan shares have whipsawed over initially conflicting statements between the company and the FDA over whether the agency had cleared the Mylan plant.

The article, which cited an internal report, said workers overrode quality controls intended to ensure the safety and efficacy of the company's prescription drugs.

The FDA inspected the plant after the article, and on July 28 the company issued a statement saying the agency found only a minor deviation. But later that day, a top agency compliance official said the investigation was ongoing and any statements to the contrary were "untrue."

Last week, the agency said it expected to wrap up the probe within a few weeks.

At the time of the article, Mylan said it periodically faces deviations in standard operating procedures and it had investigated the issue at the plant. The issue had no impact on product quality and demonstrated the company's quality systems were working, the company had said.

The FDA found that Mylan had conducted an adequate investigation, agency spokeswoman Leon said on Thursday.

Investors have worried about possible ramifications related to the FDA's probe of the plant, which is on track to produce 16 billion tablets and capsules this year. One analyst estimated it accounts for more than half of Mylan's manufacturing output.

Earlier this week, Mylan said it won approval for a generic version of a migraine drug made at the Morgantown plant, which some analysts said was a positive sign about the FDA probe.

"Clearing the manufacturing doubts should remove an overhang in the stock," Wells Fargo analyst Michael Tong said in a research note.

Mylan shares rose 3.9 percent to $14.13 early Thursday afternoon on Nasdaq.

(Reporting by Lewis Krauskopf; Editing by Derek Caney and Richard Chang)