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The federal government resorted to bullying tactics when it ordered an investigation of Humana -- one of the country's biggest private insurers -- for its decision to send customers a letter alerting them about pending health reform legislation, a leading Republican charged Wednesday.

U.S. health officials launched the probe after the Louisville-based company mailed a letter to patients enrolled in its Medicare Advantage plans -- private options that replace standard Medicare -- warning that President Obama's health overhaul could eliminate important benefits of the program.

Humana said in its letter that if Medicare Advantage funding gets cut, "millions of seniors and disabled individuals ... could lose many of the important benefits and services that make Medicare Advantage health plans so valuable."

Republican Senate Minority leader Mitch McConnell blasted the investigation of Humana on Wednesday, calling it a "federal gag order" that seeks to silence a health provider that disagrees with the administration. McConnell said he's called for a complete legal justification of the probe.

"This is so clearly an outrage," McConnell said on the Senate floor. "For explaining to seniors how legislation might affect them, the federal government has now issued a gag order on that company, and any other company that communicates with clients on the issue, telling them to shut up -- or else.

"This is precisely the kind of thing Americans are worried about with the administration's health care plan. They're worried that government agencies which were created to enforce violations even-handedly will instead be used against those who voice a different point of view," he said.

The investigation was first suggested by Senate Finance Committee Chairman Max Baucus, whose committee has jurisdiction over Medicare. The Centers for Medicare & Medicaid Services (CMS) -- which officiates over the Medicare program for seniors and Medicare Advantage options -- ordered a "cease and desist" order on all of Humana's health care mailings until the investigation is concluded.

Baucus has called the Humana letter a "scare tactic" meant to distort the current reforms under consideration. The CMS alleges that Humana's letter may have violated federal regulations, but the information distributed by the health provider was supported by the nonpartisan, independent analysis of the Congressional Budget Office.

Obama has insisted that despite planned cuts to Medicare providers, seniors would not see their benefits reduced under a health care overhaul. But CBO Director Douglas Elmendorf contradicted that Tuesday under questioning by Finance Committee Republicans, saying seniors in the private Medicare Advantage plans could see reduced benefits under Baucus' legislation. Proposed changes "would reduce the extra benefits that would be made available to beneficiaries through Medicare Advantage plans," Elmendorf said.

Humana spokesman Jim Turner said Wednesday that the company is cooperating with CMS in its investigation. But, Turner added, "We also believe Medicare Advantage members deserve to know the impact that funding cuts of the magnitude being discussed would have on benefits and premiums."

A Republican aide told FOXNews.com that the investigation is a clear breach of First Amendment rights and said the Republican leader is asking the CMS to provide legal justification for its investigation. The aide said CMS's investigation follows a pattern of intimidation put forth by the administration for any kind of dissent in the health care debate.

The Democratic Senatorial Campaign Committee lashed out at McConnell's charges Wednesday, saying, "If there was ever any doubt who Republicans are looking out for in the health care debate, Mitch McConnell has offered conclusive proof: the insurance companies.

"Republicans jeopardize their own credibility when they choose to defend big insurance companies trying to make false claims about senior citizens," the DSCC said in a press release.

Nine months into Obama's administration, no administrator of the CMS has been named -- leading some Republicans to question whether the White House had a direct hand in silencing Humana.

While the administration referred all questions about the investigation to CMS, White House spokesman Reid Cherlin said the confirmation of a CMS administrator "is a priority for the administration." CMS did not immediately answer requests for comment.

House Ways and Means Committee ranking member Rep. Dave Camp, R-Mich., has called on CMS to provide additional information on the so-called gag order, including the person or persons who authorized the agency to issue it.

"I have never seen anything like this and I question if politics was the deciding factor," Camp said in a press release. "Given that the administration has failed for more than eight months to nominate a director for CMS, I wonder if undue political pressure may have been applied on the CMS staff.

"It is Congress' responsibility to find out the facts and protect the interests of the American people. We need to know who contacted CMS, when they did it and what was said," he said.

FOX News' Chad Pergram and the Associated Press contributed to this report.