The Food and Drug Administration said Thursday that it monitored the personal e-mails of employees who had concerns about unsafe medical devices beginning in April 2010 but said it did so to investigate allegations that the employees had leaked confidential information to the public.

The FDA’s statement came in response to a Washington Post article last month that reported that the FDA 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.

The statement comes as congressional inquiry into the ­e-mail surveillance has widened. In a letter Thursday 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 Post reported that the e-mail surveillance began as early as January 2009. Documents obtained by the FDA employees — all doctors and scientists who worked for the Office of Device Evaluation— showed contents of several e-mails from January 2009 between one scientist and a congressional aide.

Those e-mails were attached to a letter sent by an FDA official to another agency as evidence why the FDA could not allow the scientist to continue to work in its organization. “We simply cannot trust him,” the letter said. The e-mails were obtained from the Equal Employment Opportunity Commission, which had collected documents in an investigation of the employee’s complaint of retaliation.

“In fact, our targeted monitoring of email content of these individuals was not initiated until April 2010, when it was brought to our attention by a company that confidential and proprietary information had been leaked to the public,” FDA spokesman Erica Jefferson said. “Our monitoring was designed to determine whether confidential information had been inappropriately released.”

The employees, who accessed their Gmail and other personal e-mail accounts from work computers, filed suit in U.S. District Court in Washington last month, alleging 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 alleges.

Jefferson also stated that whenever employees log onto the FDA computer system, they are warned that their use of the system may be monitored, intercepted, or recorded in any manner and disclosed to authorized personnel. She said that they are required to affirmatively consent to such monitoring.

The FDA has a warning, 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 said 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 monitoring’s extent and methods, as well as the legal justification.

A similar investigation 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, if the FDA obtained passwords to the employees’ personal e-mail accounts, allowing their communications on private computers to be intercepted.

The employees also communicated with Issa’s staff in 2010 and 2011, said Stephen M. Kohn, a lawyer representing the group. “We are fearful that the FDA also illegally intercepted confidential communications between the whistleblowers and Congressman Issa’s staff,” he said.

“We hope that Congress holds those responsible for illegally intercepting communications to Congress fully accountable,” Kohn said. “These interceptions have a chilling effect on all whistleblowers.”