Then-congressman Aaron Schock speaks to reporters in Peoria, Ill., in February 2015. (Seth Perlman/Associated Press)

Federal agents investigating former congressman Aaron Schock (R-Ill.) turned a congressional aide into an informant and had him record conversations surreptitiously in Schock’s district office — an aggressive tactic that legal analysts say tests the boundaries of the Justice Department’s authority when it comes to cases involving the legislative branch.

The staffer also took documents from Schock’s office — including receipts from another staffer’s desk and emails from another staffer’s account — and turned them over to investigators, court records show.

Schock, whose Capitol Hill office was decorated in the style of the TV show “Downton Abbey” and whose six-pack abs landed him on the cover of Men’s Health magazine, was indicted last year on charges that he misspent government and campaign money for his personal benefit. He has vowed to fight the case and asserted that the allegations against him are politically motivated.

Schock’s attorneys asked a federal judge to force the Justice Department to turn over to them more information on the staffer-turned-informant, who they said committed “improper — if not outright illegal — acts.” Federal prosecutors fired back Tuesday that the tactic was legal and approved by Justice Department leadership.

In a 62-page filing, prosecutors said Schock had from the beginning of the investigation “engaged in an increasingly aggressive search for some governmental misconduct claim, initially to forestall the indictment, and now to avoid the trial on the merits.”

“Public officials, including Congressman, are not immune from legitimate law enforcement methods,” prosecutors wrote.

The dust-up is reminiscent of a battle over whether the Justice Department’s controversial search of Democratic Rep. William J. Jefferson’s Capitol Hill office was legal. In that case, a federal appeals court in D.C. ruled in 2007 that FBI agents had violated the Constitution because they had viewed material that might have been protected under the “speech or debate” clause, which is supposed to curb the executive branch’s ability to investigate and prosecute congressmen for legislative activity.

“We would be deeply troubled by the Justice Department planting a bug in a congressional office,” said Jonathan Turley, a George Washington University Law School professor who specializes in constitutional law and whom Schock’s defense team asked to provide his opinion on the case. “This is actually far worse. You have someone moving around like a human surveillance device.”

In addition to recording conversations, the informant ferreted out other people’s documents, including travel-related receipts that he took from another staffer’s desk drawer and thousands of emails that he took from another staffer’s account, defense attorneys alleged. Prosecutors wrote that the informant, an officer manager, had access to other people’s documents and even email accounts as part of his job.

The informant was not named but seems to be Bryan Rudolph, based on congressional records and the job description provided in court records. Rudolph could not be reached, and a relative did not return a message seeking comment for this story.

The recordings the informant made seemed to produce unflattering material about Schock, but defense attorneys asserted that they did not give prosecutors the “smoking gun” they might have been hoping for.

During one conversation, Schock predicted to the office manager that one staff member “will never say that I told her to do what she did” and that another “will never be able to say that I told her what to do,” court records show.

Prosecutors wrote that another person recorded a phone call with Schock and that Schock referred to campaign accounts as “3 million of my money.”

Defense attorneys alleged that the staffer might have run afoul of the “speech or debate” clause. He implied he had the same attorney as other staffers and therefore may have violated attorney-client privilege with his recordings, the defense said.

They also allege that his taking of the documents amounted to theft. They said the government met with the staffer almost daily to provide instruction, receive documents and discuss scenarios and that the government was still withholding important information.

One of his handlers, for example, claimed that she had “deactivated” the phone that contained original text message exchanges with the staffer and that she could no longer remember the password, defense attorneys wrote.

“One can only wonder what criminal charges an aggressive prosecutor would bring if any ordinary citizen took steps to make highly relevant information unavailable during an investigation,” they wrote.

Prosecutors fired back that they had turned over more material than necessary, including information on how they had given the informant instruction to avoid running into attorney-client privilege or “speech or debate” problems.

Kelly B. Kramer, a white-collar criminal defense attorney at the Mayer Brown law firm, said it was not uncommon for prosecutors to use staffers as witnesses or informants, nor was it unheard of to persuade them to wear a wire. But doing so inside a congressional office, he said, might cause prosecutors to “run into some potential speech or debate issues” and that directing a staffer to go into others’ desks could be even worse.

“The thing that would strike me as more problematic, if you’re using a staffer and you’re directing them to get other people’s material, that’s fraught with peril,” Kramer said.

But, Kramer said, such misconduct might not be fatal to the case against Schock. Judges, he said, are generally sympathetic if the government employs a taint team — as they said they did with Schock — to shield investigators from material they should not be allowed to see.

Kramer said that in the case of former congressman Rick Renzi (R-Ariz.), whom he represented, a judge threw out all the wiretap evidence after learning the Justice Department had recorded the calls of defense attorneys — a “pretty big sanction” but one that stopped short of dismissing the entire case. Renzi was ultimately convicted.

Prosecutors wrote that while they believe the informant obtained the documents legally, they did not present them to grand jurors nor did they intend to use them at trial. Prosecutors also used subpoenas to get documents from Schock’s office, and they noted that he had raised no specific claims that any material they received was protected by the “speech or debate” clause.