Congress pressed the Energy Department on Thursday for answers over allegations that it retaliated against employees who spoke to a watchdog about discriminatory hiring processes at a federal nonprofit agency within the department’s jurisdiction.

Admonishing officials during an oversight hearing, Rep. Darrell Issa (R-Calif.) said Bonneville Power Administration employees were ordered by top-level Energy Department leaders “not to speak with anyone outside of BPA” about the division’s reportedly discriminatory hiring process.

“Let me be perfectly clear, retaliation against whistleblowers is illegal, and I will not stand for it,” Issa, chairman of the House Oversight and Government Reform Committee, declared in his opening statement.

In a July 2013 Energy Department inspector general’s “management alert,” the BPA was found to have “engaged in prohibited personnel practices in 65 percent (95 to 146 cases) of its competitive recruitments conducted from November 2010 to June 2012.” Practices “appeared to have effectively disadvantaged veterans and other applicants.”

In response, the BPA fired one employee who aided in the alert and suspended or reassigned nearly a half-dozen others.

The agency discharged two top BPA officials last month: Administrator Bill Drummond and Chief Operating Officer Anita Decker.

“In regard to veterans, we made a regrettable mistake,” Decker said in her opening statement. “We stopped making the mistake over a year ago, and I want to be part of making this right.”

The department remained under investigation after another BPA official told lawmakers that a top Energy Department leader ordered staff members to not talk.

Committee members asked Inspector General Gregory H. Friedman whether the department’s deputy secretary, Daniel B. Poneman, issued those instructions.

“We have no allegations that Mr. Poneman is muzzling,” Friedman said.

Poneman, also testifying, denied Issa’s accusations.

“I don’t know what information he based that comment on,” Poneman said of Issa after Rep. Gerald E. Connolly (D-Va.) asked him to respond.

“I told [BPA acting administrator Elliot Mainzer] it was imperative that everybody understand clearly that there can be no retaliation, no reprisal, no adverse action taken to anybody stepping forward and expressing their concerns in any case,” Poneman said.

The GOP-dominated committee has been sharply critical of the Energy Department’s initiatives. Poneman, who was appointed by President Obama in 2009, faced tough questioning from the panel in 2011 when the Energy and Labor departments were called to justify the administration’s subsidizing of the clean-energy sector.

Rep. Jim Jordan (R-Ohio) asked Poneman about his tenure on the department’s credit review board after its support of failed solar-panel manufacturer Solyndra.

But the committee’s ranking Democrat, Rep. Elijah E. Cummings (Md.), interrupted, calling Jordan’s questioning irrelevant.

In his final comments, Poneman spoke passionately of his support of whistleblowers, comments that appeared to at least quiet his critics.

Issa closed the hearing saying that he hoped the investigation would produce closer scrutiny in all government agencies.