Hillary Rodham Clinton, photographed on Nov. 21, 2014, in New York. (Bebeto Matthews/AP)

The revelation that Hillary Rodham Clinton used a private e-mail system as secretary of state has revived a Senate investigation into another aspect of her tenure.

Sen. Charles E. Grassley (R-Iowa), who heads the Judiciary Committee, had previously questioned Clinton’s use of a program that allowed some political allies to work for the government while pursuing private-sector careers.

Grassley had sought e-mails and other documents from the State Department.

But he didn’t know until last week that Clinton was exclusively using a private e-mail account that could contain relevant information about her use of the so-called “special government employee” program. Huma Abedin, a Clinton confidante and adviser who was granted the special designation, also used the private e-mail system.

Grassley has in recent days renewed his effort to get answers from the State Department. That opens a second line of inquiry on Capitol Hill into the Democrats’ presumptive presidential front-runner, who was already facing an inquiry from a House committee seeking her e-mails related to the U.S. response to an attack on the U.S. diplomatic post in Benghazi, Libya, in 2012.

In a tense exchange with reporters, State Department deputy spokeswoman Marie Harf denied anything inappropriate occurred after revelations that former secretary of state Hillary Clinton used a private e-mail account for work. (C-SPAN)

Grassley and other critics have said Clinton’s use of the special employee program appeared unusual.

State Department records show that a half-dozen of Clinton’s political allies were granted the special designation during her tenure. Not all of them received government salaries. But critics say abuse of the program could give private-sector firms unfair access to government information and internal deliberations.

Spokesmen for Clinton and the State Department say her use of the program was appropriate and followed government rules. More than 100 people, most with expertise in niche areas of science and global affairs, received the designation each year.

Asked to respond directly to Grassley’s concerns that Clinton may have misused the program, Clinton spokesman Nick Merrill replied by e-mail: “With all due respect, we disagree.”

No one knows how widely the program is used across the government. Grassley last year asked the Government Accountability Office to conduct a review. The assessment is not complete, but in the meantime, Grassley has focused his attention on Clinton and some of those who received the special designation under her leadership.

Some recipients were political advisers with limited State Department expertise. Others, Grass­ley said, appeared to have turned the program on its head: Instead of being outside experts brought in to assist the government, they were State Department employees who launched secondary careers in the private sector while remaining tied to the department.

“The public’s business ought to be public with few exceptions,” Grassley said in a statement Saturday. “When employees are allowed to serve the government and the private sector at the same time and use private email, the employees have access to everything and the public, nothing.”

Hillary Clinton’s private e-mail address that she used while secretary of state reinforces everything people don’t like about her, argues The Post’s Chris Cillizza, and is very dangerous to her presidential ambitions. (The Washington Post)

Grassley said he will press the State Department in the coming weeks “to answer for any blurring of the lines between public and private service and any concealing of the blurred lines through private email.”

Questions about Clinton’s use of the special program were first raised in 2013, when it became public that Abedin was being paid by the State Department while also working for an international consulting firm with close ties to Bill and Hillary Clinton.

Through a request under the Freedom of Information Act, several news organizations, including The Washington Post, have since learned the extent to which Hillary Clinton used the program.

Others granted the special status included a former campaign manager, a longtime legal and personal adviser, a former House member now affiliated with a group backing a Clinton presidential bid, a former pollster and others who have supported the Clintons in their political and philanthropic organizations.

In interviews, State Department officials and several of the individuals said the special government status was legitimate and had no relationship to Hillary Clinton’s political ambitions. Some said they declined compensation for their work under the special status.

Aside from Abedin, Clinton political allies who were granted the special status included Maggie Williams, Clinton’s 2008 campaign manager; Jeremy Rosner, a former Clinton aide; Jonathan Prince, a speechwriter for Bill and Hillary Clinton; Kathleen Kennedy Townsend, a former Maryland lieutenant governor who is on the board of American Bridge, a left-leaning political operation that has defended Hillary Clinton against partisan attacks; and Cheryl Mills, a former White House deputy counsel and longtime adviser to Hillary Clinton.

Williams received the special status to work on issues relating to women and girls, State Department spokesman Alec Gerlach said. Williams did not respond to requests for comment.

State Department documents show that Mills received no compensation for her work, which was tied to reconstruction efforts in Haiti. Mills did not respond to requests for comment. Merrill noted that Mills retained her affiliation with the department to work on Haiti after Clinton left.

Prince also received no compensation for his work, which he said was tied to Middle East peace talks and unrelated to anything involving the Clintons. Townsend, who did not receive a government paycheck from her special designation, did not respond to requests for comment.

Ellen O. Tauscher, a former California congresswoman, and Caitlin Klevorick, who previously worked at the Clinton Foundation, launched activities in the private sector while working as special government employees at the State Department.

Records show that Tauscher received a special designation in 2012 after serving as special envoy for strategic stability and missile defense in the office of the secretary of state.

Tauscher said she served only a few months. During that time, she joined a corporate board. She later started work as a lawyer with a Washington firm.

Tauscher, who has since been active in the Ready for Hillary Super PAC, said she received the designation after expressing interest in retiring from government. She was told that the State Department wanted her to continue to serve at least part time so she could remain involved with ongoing missile-defense negotiations with Russian officials.

Klevorick, who joined the State Department as an aide to Clinton, received two one-year special appointments beginning in January 2012. During that time, records show that Klevorick established CBK Strategies, a consulting firm that advises government and corporate clients on communications and policy.

Klevorick, whose participation in the program was first reported last year by Pro Publica, did not respond to requests for comment. Gerlach said Klevorick served as a senior adviser to the department.

Rosner, a pollster who worked for the National Security Council during Bill Clinton’s administration, had a special government employee assignment in 2011, a period during which he continued his association with the Washington-based political consulting firm Greenberg Quinlan Rosner. Rosner said his work at State, on a public diplomacy project in Pakistan, was unrelated to the Clintons.

Abedin said she left full-time employment in 2012 after giving birth to a son with her husband, former representative Anthony Weiner (D-N.Y.), who was running for mayor of New York. She said in a letter to the State Department that the change was approved by State Department legal staff.

In the letter Abedin wrote to the State Department in 2013, she said her work at the consulting firm Teneo was unrelated to anything involving the department.

Rosalind S. Helderman, Carol D. Leonnig and Alice Crites contributed to this report.

Correction: An earlier version of this article incorrectly identified Jeremy Rosner as a former Clinton pollster. Rosner, now a Democratic pollster, worked in the White House under President Bill Clinton in 1993 and 1994 as a National Security Council counselor and director of legislative affairs. He has done no polling for Bill or Hillary Clinton.