A senior Senate Republican has launched an investigation into the Food and Drug Administration’s secret e-mail monitoring of scientists who warned that unsafe medical devices were approved over their objections, saying whistleblowers often are treated “like skunks at a picnic.”

In a five-page letter released Wednesday, Sen. Charles E. Grassley (R-Iowa) demands that FDA Commissioner Margaret A. Hamburg disclose who authorized the monitoring, how many employees were targeted and whether the agency obtained passwords to their personal e-mail accounts, allowing their communications on private computers to be intercepted. Grassley also wants to know whether the monitoring is still going on.

The letter came in response to Monday’s Washington Post report on a lawsuit filed in U.S. District Court in Washington last week by six current and former FDA scientists and doctors. They allege that the government violated their constitutional privacy rights by monitoring personal communications on Gmail they said were lawful.

“I write to express my concerns over your agency’s treatment of whistleblowers as a result of their disclosures to Congress, and specifically disclosures to my office,” Grassley, ranking member on the Finance Committee, told Hamburg. “Whistleblowers ... are often treated like skunks at a picnic.”

FDA spokeswoman Erica Jefferson said the agency “will respond directly” to the senator about his letter. She declined further comment. Documents the whistleblowers obtained under the Freedom of Information Act showed that the agency appeared to believe reviewers had disclosed confidential information about the machines. They deny sharing information improperly.

Grassley and his staff have investigated the FDA for years as controversies over medical device and drug safety have swept through the agency. He is a longtime advocate for protections for whistleblowers.

The doctors and scientists, five of them responsible for reviewing devices for cancer screening and other purposes and one a branch manager, had warned Congress and the White House that the FDA overruled them in approving several radiological machines they believed posed unacceptable risks to patients, government documents show.

In response, the agency began a two-year surveillance campaign, intercepting the reviewers’ personal e-mail communications on their work computers with one another, congressional staffers, the Equal Employment Opportunity Commission and the Office of Special Counsel, documents show.

The plaintiffs contend the monitoring was a blatant form of retaliation. Information gathered this way eventually contributed to the harassment or dismissal of all six workers, the suit alleges.

Grassley warned Hamburg that it is illegal for a government agency to interfere with an employee’s right to air concerns to members of Congress. He said the FDA had “no evidence” that one of the device reviewers, Paul J. Hardy, disclosed confidential business information about the devices, yet fired him after he exchanged Gmail messages with congressional staff, including an investigator who worked for Grassley.

“It is troubling to me to see your Agency actively pursue the dismissal of an employee ... not because they violated procedure and leaked genuinely confidential classified information, but simply because you “cannot trust him,” Grassley wrote. He told Hamburg the FDA’s retaliation against Hardy “directly contradicts” testimony she gave during her 2009 confirmation hearings of her intent to protect whistleblowers by “creating a culture that enables all voices to be heard.”