In setting an April 12 deadline, U.S. District Judge Emmet G. Sullivan created the prospect that key Hillary Clinton aides would face questions just as she tries to secure the Democratic nomination. (Melina Mara/The Washington Post)

A federal judge ruled Tuesday that top aides to Democratic presidential front-runner Hillary Clinton should be questioned under oath about her use of a private email server as secretary of state, raising new political and legal complications for Clinton as she tries to maintain momentum for her campaign.

The ruling granted a request from the conservative group Judicial Watch, which sought testimony from State Department officials and members of Clinton’s inner circle to determine whether Clinton’s email arrangement thwarted federal open-records laws.

In setting an April 12 deadline for Judicial Watch and the government’s lawyers to lay out a plan to proceed, U.S. District Judge Emmet G. Sullivan created the prospect that key Clinton aides would face questions just as she tries to secure the Democratic nomination and pivot to a hotly contested November general election.

The judge said that months of piecemeal revelations to date about Clinton and the State Department’s handling of the email controversy created “at least a ‘reasonable suspicion’ ” that public access to official government records under the federal Freedom of Information Act was undermined.

“There has been a constant drip, drip, drip of declarations. When does it stop?” said Sullivan, a 1994 Bill Clinton appointee who has overseen several politically sensitive FOIA cases. “This case is about the public’s right to know,” he said.

There are at least three ongoing investigations into Democratic presidential candidate Hillary Clinton's time as Secretary of State. Here's an explanation of who is investigating, and why. (Gillian Brockell/The Washington Post)

Clinton has struggled on the campaign trail to move past the issue of her email use. Republicans have made clear that they will make the subject a centerpiece of attacks against her should she win the nomination, with Marco Rubio saying her use of a private server “disqualified” her to be president and Donald Trump telling Fox News this week that she “seems to be guilty” of a crime before amending that to say it would be something he would look into if elected.

Clinton also confronts an FBI investigation and a State Department inspector’s general probe into whether classified information was mishandled or other federal laws were violated.

“This is one of several lawsuits filed by the same right-wing group, which will stop at nothing in pursuing the Clintons, just as they have done since the 1990s,” said Clinton campaign spokesman Nick Merrill.

The Judicial Watch lawsuit came over a May 2013 request for information about the employment arrangement of Huma Abedin, a longtime confidante who served as Clinton’s deputy chief of staff.

For six months in 2012, Abedin was employed simultaneously by the State Department, the Clinton Foundation, Clinton’s personal office and a private consulting firm connected to the Clintons.

Judicial Watch has indicated that it intends to seek sworn testimony from Abedin as well as Cheryl D. Mills, who was Clinton’s chief of staff at State. Both aides are especially close to Clinton and would likely play roles in her White House if she won the presidency.

On Tuesday, Sullivan said he might also at some point direct the State Department to subpoena Clinton, Abedin or others to return all of the emails sent and received from Clinton’s personal email system, not merely those they or their attorneys have deemed work-related and submitted to the State Department.

The suggestion was an ominous sign for Clinton that 31,000 emails she has said were purely personal and deleted could make their way into the public realm.

Officials with the State and Justice departments declined to comment, citing ongoing litigation. Discovery orders are not readily appealable. An attorney for Abedin declined to comment.

In a statement, Judicial Watch President Thomas J. Fitton called the ruling “a major victory” for the public and did not rule out that Clinton could become one of several current and former department officials to be questioned.

“While Mrs. Clinton’s testimony may not be required initially, it may happen that her testimony is necessary for the Court to resolve the legal issues about her unprecedented email practices,” Fitton said.

Fitton’s group in court filings did not ask to depose Clinton by name, but it targeted its requests at individuals who handled her transition, arrival and departure from the department and who oversaw Abedin, a direct subordinate.

The group singled out Undersecretary for Management Patrick F. Kennedy; John F. Hackett, director of information services; Executive Secretary Joseph E. Macmanus; Mills; Abedin; Clinton lawyer David E. Kendall; and Bryan Pagliano, a Clinton staff member during her 2008 presidential campaign who helped set up the private server.

Judicial Watch also named Clinton technology contractors Platte River Networks and Datto Inc. and asked to subpoena archived copies of sent or received emails on the private server.

Sullivan noted that there was no dispute that senior State Department officials were aware of the email setup from when Clinton took office, citing a January 2009 email exchange including Kennedy, Mills and Abedin about establishing a “stand-alone network” email system.

Sullivan also said the State Department’s inspector general last month faulted the department and Clinton’s office for overseeing processes that repeatedly allowed “inaccurate and incomplete” FOIA responses, including a May 2013 reply to one that found “no records” concerning email accounts that Clinton used, even though dozens of senior officials had corresponded with her private account.

Sullivan called the situation unprecedented, “very, very troubling,” an “enormous waste of resources.”

“Is this where we are now? The government has to ask an employee to return documents? It can’t demand documents?” Sullivan said. “I’m not sure how much comfort that would give the public. . . . It boggles the mind.”

Justice Department lawyers argued in a two-hour hearing that the State Department is poised to finish publicly releasing all 54,000 pages of emails that Clinton’s attorneys determined to be work-related and that were returned to the State Department at its request for review.

The State Department has been releasing Clinton’s newly recovered correspondence in batches since last summer, with a final set due Monday.

Meanwhile, former Clinton department aides Mills, Abedin, Jacob Sullivan and Philippe Reines have returned tens of thousands of pages of documents to the department for FOIA review, with releases projected to continue into at least 2017.

The State Department also has asked the FBI to notify it of any of an estimated 31,000 deleted emails deemed personal by Clinton’s attorneys that the FBI is able to recover in its investigation of the security of the private email server, and to preserve them.

“State has reached out to the individuals. State has reached out to the FBI. State has really done everything that it can to get these records,” said Justice Department attorney Steven A. Myers.

Myers argued that FOIA requires the agency to release records only under its control — not under the control of its current or former officials — and that “federal employees routinely manage their email and ‘self-select’ their work-related messages” when they delete personal emails from government email accounts.

The case before Sullivan is one of more than 50 active FOIA lawsuits by legal groups, news media organizations and others seeking information included in emails sent to or by Clinton and her aides on the private server she used as secretary from 2009 to 2013.