Congressional investigators wanted to talk.

Rep. Rashida Tlaib, the freshman Democrat from Michigan, faced allegations that she improperly paid herself a salary from her campaign account, including a bulk payment of $15,500 after the election was over.

Tlaib told reporters the payments were proper. But when the Office of Congressional Ethics, the House’s independent, nonpartisan watchdog, asked to interview her, the congresswoman refused. So did her staffers who had been involved with the payments.

Tlaib, who resides on the progressive wing of her party, isn’t alone in this response when OCE came calling. Other lawmakers who stonewalled include a Virginia Republican who allegedly sent his House staff on personal errands, including picking up milk and caring for his dog, and a Freedom Caucus lawmaker from North Carolina who continued to pay his chief of staff even after barring him after accusations of sexual harassment.

Indeed, House members and staffers of both parties are increasingly dodging ethics investigators. The last decade showed a sharp drop in cooperation starting in mid-2016. Before that, in 74% of distinct cases subjects cooperated fully, providing interviews and documents as requested, according to a ProPublica review of every case in which OCE found a potential violation. Since then, full cooperation has plummeted to just 33% of cases.

Today, it’s common for lawmakers from both parties to refuse not just some requests for interviews and documents from OCE, but all of them. In the last four years, subjects in 11 of 18 distinct cases refused any cooperation whatsoever. In the six years before that, there were just three such cases out of 43.

Congress created OCE, which has a 10-person staff, in 2008 after a string of scandals on the Hill cast doubt over whether the House ethics committee, led by lawmakers, could effectively root out corruption in its own ranks. OCE, however, was not granted subpoena power, so it still relies on the committee to compel interviews and documents from uncooperative subjects.

The reasons offered for the increased obstruction vary. Despite lacking subpoena power, OCE is perceived as more aggressive, so members may prefer trying their luck with the committee. The committee has a reputation for moving more slowly, being more deferential to members and being willing to resolve some matters confidentially. One case, involving alleged financial improprieties by Rep. David Schweikert, R-Ariz., has languished in the committee for almost two years. (He has defended his actions.)

In addition, some observers suggest that President Donald Trump has licensed resistance to outside scrutiny, as officials from his White House have ignored congressional subpoenas and the president himself refused to sit down for an interview with special counsel Robert Mueller.

The drop in cooperation represents the latest attempt by lawmakers of both parties to push back on the watchdog. In 2010, Democrats from the Congressional Black Caucus tried unsuccessfully to curtail OCE’s ability to publicize its findings. In 2017, after Trump was elected, House Republicans attempted to limit OCE’s independence, but they failed after the move drew widespread outrage.

Unable to legislate changes to its mission, some lawmakers have figured out a way to try to do an end-run around the body. Unlike past efforts, the refusal to cooperate requires no rule changes, and it has been quietly playing out on a case-by-case basis.

Craig Holman, a veteran observer of congressional investigations and a lobbyist for the watchdog group Public Citizen, said ProPublica’s review showed “a breathtaking shift in cooperation rates.”

“This shift vastly bolsters the need to grant OCE with subpoena authority,” he said.

Omar Ashmawy, director of OCE, acknowledged a recent decline in cooperation by subjects in cases referred to the committee for further review. “Perhaps due to the advice of some counsel,” he said.

The pool of Washington lawyers who represent lawmakers before OCE is small.

In interviews with ProPublica, several of those lawyers, representing both Republicans and Democrats, said they generally believe their clients are better off dealing with the committee instead of OCE.

Robert K. Kelner, a Republican attorney at Covington & Burling, said the shift against cooperating was due to OCE tactics he called aggressive and unfair.

“There’s a strong view among the bar of lawyers who practice before the ethics committee that the ethics committee follows a much more fair process than OCE,” he said. “If you have a choice of waiting to have your case heard by a body that will abide by basic norms of due process, why wouldn’t you wait for that?”

Kelner lauded the committee for sometimes resolving cases privately, unlike OCE: “What we should care about is whether they’re causing members to comply with the rules, not about whether it’s done publicly or whether they make a spectacle of it.”

Among the complaints from defense attorneys about OCE is that the body ascribes guilt to lawmakers for not cooperating, accepts anonymous complaints and pressures members to make their staffers cooperate as well.

Another Washington attorney, who represents Democrats and would not speak on the record out of fear of antagonizing OCE, agreed that the committee has a tendency to move more slowly than OCE, but said that was a sign of more professional and thorough investigations.

“The incentive structures are different. OCE has more of an incentive to draw attention and make waves. Most of the time the committee is trying to figure out how to help members basically comply,” the attorney said. “They send them a private letter and ask them not to do it again, and generally it has the desired effect.”

A third ethics lawyer, who also requested anonymity for similar reasons, said when clients are guilty, or if they possess documents that implicate them, it may be in their best interest to refuse to voluntarily turn that information over to OCE.

While OCE does make its findings public in cases in which it finds potential violations — creating negative press for members of Congress — it has no authority to formally reprimand lawmakers on its own. OCE refers such cases to the committee, which then decides whether to conduct its own investigation, and ultimately whether to discipline lawmakers.

“Even if the stakes are higher with the ethics committee, you probably have a better shot of having it shut down,” the lawyer said.

In 2016, OCE referred the case of Rep. Roger Williams, R-Texas, who owns a car dealership and pushed a provision to allow car dealerships to continue renting out vehicles subject to safety recalls.

Williams did not cooperate with OCE, refusing to provide documents or sit down for an interview. In its report, OCE concluded “there is substantial reason to believe that Representative Williams’ personal financial interest in his auto dealership may be perceived as having influenced his performance of official duties.”

But the committee decided not to take action against Williams. Though it found he “may have had some personal financial interest in the adoption of his amendment,” the committee dismissed any such financial interest as “speculative and hypothetical.” How much loaning cars out increased the lawmaker’s profits, and how much banning recalled cars would hurt his profits, was “unclear, if not impossible to quantify.”

Advocates for strict ethics enforcement blasted the decision by the committee, with one saying it “proves they are an ‘ethics’ committee in name only.” A spokeswoman for Williams said he “followed the rules as set forth by Congress” and noted that he “fully participated” with the committee.

The committee’s investigation into the allegations against Tlaib, which it received a referral on last summer, is ongoing. In the case of the Virginia Republican, Rep. Tom Garrett, who left office last year, the committee ultimately found he did send his staff out on personal errands. But it determined it could not fine him because he was not seeking reelection. The committee did reprimand the Freedom Caucus lawmaker from North Carolina, Rep. Mark Meadows, who continued to pay his chief of staff even after banning him from the office after multiple accusations of sexual harassment. Meadows was required to cover the $40,000 cost. (On Friday, Meadows was named Trump’s new chief of staff.)

Schweikert, Tlaib, Garrett and Meadows did not immediately respond to requests for comment. An official with the House ethics committee declined comment.

A former OCE official attributed the drop in cooperation, in part, to the 2016 federal charges against David Bowser, chief of staff to a Republican congressman, over false statements he allegedly made to OCE. Bowser attempted to obstruct OCE’s investigation into whether a campaign consultant was paid with taxpayer funds, prosecutors alleged, by failing to produce documents, influencing the testimony of witnesses and falsely claiming that the consultant was solely hired to work on congressional matters. Bowser was convicted of making false statements in March 2018. His lawyer, Leslie McAdoo Gordon, said she advises against cooperating with OCE and believes the Justice Department overreached in Bowser’s case.

“That may be why you have members trying to be very careful not to be put in that same situation,” said Kedric Payne, former deputy chief counsel at OCE.

But Payne said refusing to cooperate was “not a winning strategy” — forcing OCE to make judgements without hearing the lawmaker’s side of the story and only increasing the odds that OCE would refer the case to the committee for further review, thus making the allegations public.

Ashmawy noted that for cases that OCE decided should be dismissed, which are not made public, the vast majority of subjects had cooperated.

Ashmawy suggested that even when lawmakers do not cooperate, OCE investigations go on. Investigators can get the documents they need — including texts, emails and financial records — from third parties, such as former staffers and businesses.

“While many have suggested that OCE needs subpoena authority, we do quite well without it,” Ashmawy said. “And that’s a credit to the office.”