Congressional inquiry into the Food and Drug Administration’s e-mail surveillance of its own employees widened Thursday as a House lawmaker demanded that the agency explain the legal basis for targeting employees who were raising concerns about unsafe medical devices.

In a letter to FDA Commissioner Margaret Hamburg, House Oversight and Government Reform Committee Chairman Darrell Issa (R-Calif.) warned that the FDA’s monitoring of personal communications between FDA doctors and congressional staff was “unlawful and will not be tolerated.”

The letter sent Thursday came in response to a Washington Post article last month reporting that the FDA for two years intercepted and stored the Gmail communications of a group of agency doctors who raised concerns with Congress about the agency approving cancer-screening and other devices despite the doctors’ determinations that the devices were not safe or effective.

A similar probe has been launched by Sen. Charles E. Grassley (R-Iowa), whose staff communicated with the FDA doctors about their concerns. Grassley wants to know, in particular, whether the FDA obtained passwords to the employees’ personal e-mail accounts, allowing their communications on private computers to be intercepted.

Grassley spokeswoman Jill Gerber said Thursday that FDA officials have given “no indication” of when they will reply to the senator’s request and have signaled that they are unlikely to answer all of his questions.

The employees, who accessed their Gmail from work computers, filed suit in U.S. District Court in Washington last month alleging that the agency violated their constitutional right to privacy. The FDA relied on the information it gleaned through secret surveillance to fire, harass or pass over for promotion at least six doctors and scientists who communicated with Congress, the suit alleged.

FDA spokeswoman Erica Jefferson said the agency would respond directly to Issa.

The agency has warnings on its computers, visible when logging on, that employees have “no reasonable expectation of privacy” in any data passing through or stored on the system, and that the government may intercept any such data at any time for any lawful government purpose.

“In this case, the FDA’s purpose was not lawful,” Issa asserted in his letter. “FDA was not investigating wrongdoing or tracing a security breach. In fact, FDA’s purpose appears to have been unlawful because retaliation against a whistleblower is illegal.”

Issa noted that communicating with Congress is a protected form of whistleblowing.

He requested that Hamburg identify the individuals responsible for initiating the surveillance, the dates when surveillance began on each employee and the extent of monitoring and methods used, as well as the legal justification.

The monitoring began as early as January 2009, The Post reported, citing documents the employees received under a Freedom of Information Act lawsuit and from records from an Equal Employment Opportunity Commission investigation.

The employees also communicated with Issa’s staff in 2010 and 2011, said Stephen M. Kohn, an attorney for the group.

“We are fearful that the FDA also illegally intercepted confidential communications between the whistleblowers and Congressman Issa’s staff,” Kohn said. “We hope that Congress holds those responsible for illegally intercepting communications to Congress fully accountable. These interceptions have a chilling effect on all whistleblowers.”