The FBI’s only impediment, these people allege, is possible meddling by the White House to limit the investigation’s scope. Arizona Sen. Jeff Flake, a Republican whose last-minute show of leverage forced the new FBI inquiry, has told the White House and the FBI that he expects “a fulsome” investigation. Newspaper editorial boards have warned that the Trump administration’s interference in the investigation would taint the FBI’s findings.

As the FBI explores whether Supreme Court nominee Brett Kavanaugh sexually assaulted women while in high school and college, elected officials and others have suggested that federal agents can authoritatively get to the bottom of an alleged attack said to have occurred three decades ago.

“FBI agents are experts at interviewing people and quickly dispatching leads to their colleagues around the world to follow with additional interviews,” former FBI Director James Comey assured Americans in a New York Times op-ed. “Unless limited in some way by the Trump administration, they can speak to scores of people in a few days, if necessary.”

But these arguments leave out the inherent limitations of FBI investigations in cases such as this.

How Much Can We Expect From This Week’s FBI Investigation?

The FBI agents probing whether Kavanaugh sexually assaulted Christine Blasey Ford when they were both in high school lack something that most serious government investigations require: subpoena power.

Unlike agents tasked with investigating a crime, the FBI must rely on witnesses to cooperate voluntarily with its investigation. A possible witness approached by the FBI may simply slam the door in agents’ faces with no repercussions. At any time, witnesses who choose to cooperate can refuse to answer a specific question, and they won’t have to provide any explanation for their refusal.

Take, for example, Mark Judge, who Ford said was present when Kavanaugh allegedly assaulted her. Judge has reportedly cooperated with the FBI investigation, but it is impossible to determine what exactly that means without knowing more about what he’s told them. Judge’s sitting down with the FBI, confirming his name and date of birth, and then refusing to answer any other questions could technically constitute cooperation. While lying to the FBI is a felony, refusing to answer FBI agents’ questions carries no penalty.

The only way to compel someone to speak as part of this inquiry would be for the Senate to subpoena witnesses. The Republican majority appears unwilling to exercise that power.

Why Didn’t Kavanaugh’s Previous FBI Background Investigations Turn Up the Allegations?

During his Senate testimony last week, Kavanaugh pointed to “six separate FBI background investigations over 26 years” to suggest that the allegations against him are questionable and possibly politically motivated. If these claims were real, Kavanaugh seemed to be asking, why didn’t the FBI turn them up over the last two and a half decades?

It’s true that Kavanaugh would have been subjected to background investigations, given his career of government work in the Office of Independent Counsel during the Clinton administration, in the White House during the George W. Bush administration, and as a judge on the U.S. Court of Appeals for the District of Columbia Circuit. His initial background checks, including for when he served as Bush’s staff secretary, would have been done so that he could obtain security clearances. FBI background checks for judicial nominees are similar to those conducted for security clearances, but judicial background checks focus more on professional conduct and relationships, rather than a nominee’s personal life. As a result, Kavanaugh’s judicial background checks would have been even less likely to unearth allegations about him as a high school and college student than the ones he underwent as an executive branch employee.

The FBI leaves many stones unturned when conducting these checks. U.S. government employees and contractors seeking security clearances must fill out forms that include all the addresses where they’ve lived, previous employers, and financial information. FBI agents then use this information to verify the subjects’ life stories and locate possible red flags. When conducting interviews, they typically rely on people who live near or worked with the subject, and the people they contact are under no legal obligation to talk to them.

About a year ago, for example, an FBI agent knocked on my door. She said she wanted to ask me some questions about a neighbor. I knew the neighbor was an engineer for a well-known defense contractor, but until that moment, I didn’t know he worked on sensitive projects. The FBI agent said she was visiting me as part of a routine background check necessary to renew his security clearances.

I didn’t have to talk to her, but I saw no harm in it.

She asked me if the neighbor, to my knowledge, had any financial problems or exhibited behavior that otherwise concerned me.

“No,” I answered.

“Do you know him to associate with any terrorists or subversive elements?” she asked.

I laughed. “No, definitely not,” I said.

She thanked me and walked to one of the other houses in the neighborhood, no doubt planning to pose the same questions to someone else.