WASHINGTON - The FBI was again thrust to the center of the day’s biggest political controversy Friday when Republican Senate leaders agreed to allow the bureau to conduct a limited, additional background check of Brett Kavanaugh, whose nomination to the Supreme Court hangs in the balance over allegations of sexual misconduct.

President Donald Trump said in a statement that he had ordered the bureau to "conduct a supplemental investigation to update Judge Kavanaugh's file" and added that the update "must be limited in scope and completed in less than one week."

The probe is considered a part of Kavanaugh's background check because the allegations are from Kavanaugh's teenage years and the activity does not involve any federal crimes, officials said. That is important because, unlike in a criminal investigation, additional work had to be requested by the White House, which is allowed to impose parameters on the scope, officials said.

From the probe of Hillary Clinton's use of a private email server to the examination of Trump's presidential campaign, the FBI has grown accustomed in recent months to conducting investigations that are viewed through sharply partisan lenses. The case of Kavanaugh, though, presents significant challenges.

The allegations against the Supreme Court nominee are decades old, and his accusers say their memories - sometimes of dates and locations - are hazy. A background check is not like a criminal investigation; investigators would not use tools such as grand juries, subpoenas and search warrants, meaning potential witnesses could simply decline to be interviewed.

The FBI will ultimately report its findings - which, in Kavanaugh's case, will probably be mostly reports of interviews - to the White House. The bureau will not conclude whether the accusers are believable or not, or tell the White House whether it should withdraw Kavanaugh's nomination.

"They're not going to crack the case," said Ron Hosko, a former assistant FBI director. "The best they can hope to do is find the facts, relay the statements of the witnesses that presumably would be later tested if Congress chose to do so and the president greenlighted it."

That is not to say an FBI investigation will not yield new information. Witnesses who have provided only brief statements to Congress might be more willing to cooperate with law enforcement. Mark Judge, for example, who is accused of having been in the room when Kavanaugh is alleged to have assaulted Christine Blasey Ford when both were in high school, is willing to answer the bureau’s questions, his attorney said Friday.

Judge and others will have a strong incentive to tell the truth. Lying to the FBI is a federal crime. FBI agents are also trained specifically to investigate such matters. Their detailed inquiries will probably reveal answers to questions in a way that partisan congressional hearings cannot.

Once the White House gives Congress the FBI's report, lawmakers might decide to hold a hearing or change their vote. If investigators uncover evidence that Kavanaugh lied to lawmakers during hearings or on his background-check forms, that could spark a criminal investigation in which law enforcement could use the full extent of its legal powers.

"Investigations are surprising," said Chuck Rosenberg, a former U.S. attorney, Drug Enforcement Administration head and FBI official. "I spent most of my life doing them. They take strange turns."

The scope of what the FBI will investigate was not immediately clear. Three women have leveled various allegations against Kavanaugh, all of which he denies. The Senate Judiciary Committee, in a news release indicating it had requested that the administration order a supplemental background investigation, said the probe would be limited to "current credible allegations against the nominee" and that it would be completed "no later than one week from today."

Only Ford, who claims that Kavanaugh assaulted her more than three decades ago at a party in suburban Maryland, has testified before Congress. Another woman, Deborah Ramirez, a classmate of Kavanaugh's at Yale University, told New Yorker magazine that Kavanaugh exposed his penis to her when both were in their first year at the school. Another, Julie Swetnick, said in a declaration that Kavanaugh was physically abusive toward girls in high school and present at a house party in 1982 where she says she was the victim of a "gang" rape.

Debra Katz, one of Ford's attorneys, said that her client welcomed the FBI investigation, adding that no "artificial limits as to time or scope should be imposed" on it.

Democrats have for nearly two weeks been calling on the bureau to look into the allegations against Kavanaugh and repeatedly pressed the nominee to support that demand at a hearing Thursday where Ford testified. Until Friday, though, Republicans and the White House had largely resisted those calls. They argued that Kavanaugh, a sitting federal judge, had been background-checked many times and asserted that an FBI investigation would not produce a conclusion on the truth of the allegations he faces.

For their part, the FBI and Justice Department had previously noted their role was not to “make any judgment about the credibility or significance of any allegation” but rather “to provide information for the use of the decision makers.” When Ford’s allegations first emerged, the bureau sent them to the White House as an update to Kavanaugh’s background-check file.

Officials had been waiting for a request from the White House before doing more work.

On Friday, just moments before a Senate Judiciary Committee vote to determine whether Kavanaugh's nomination could proceed to the Senate floor, Sen. Jeff Flake, R-Ariz., declared it "would be proper" to postpone the floor vote for "up to, but not more than, one week" so the FBI could conduct an investigation. He said that investigation should be limited "to the current allegations that are there." The statement came amid growing calls, including from the American Bar Association, for senators to slow down and let the FBI go to work.

Flake's call was soon endorsed by two others considered swing votes on the nomination - Sens. Lisa Murkowski, R-Alaska, and Joe Manchin III, D-W.Va., - and then the Republican leadership. Trump said he would defer to them. "They have to do what they think is right," Trump said.

Flake still voted to move Kavanaugh's nomination to the Senate floor, though it remains unclear now when the chamber might vote on it. Some current and former law enforcement officials said they were skeptical an FBI investigation would quell the controversy and worried that it would simply provide for more controversy surrounding the bureau.

"I would love to know the truth, and maybe the truth would set us all free," Hosko said, "but it may not be knowable right now."

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