WASHINGTON — The FBI barely glanced at potentially crucial evidence in its investigation of Hillary Clinton’s use of a private email server while secretary of state, according to a portion of a watchdog report made public Monday.

A confidential source gave thumb drives to the FBI that contained State Department data acquired via cyber intrusions — including emails from President Barack Obama and others, according to a declassified appendix to a June 2018 Justice Department inspector general report.

But the feds declined to “comprehensively” analyze those drives due to concerns about individual data caught up in the hack — despite an internal draft memo concluding it was necessary to “assess the national security risks” pertaining to Clinton’s private server use.

“This document shows an extreme lack of effort and due diligence in the FBI’s investigation of former Secretary Clinton’s email usage and mishandling of highly classified information,” said Senate Judiciary Committee Chairman Chuck Grassley (R-Iowa) of the appendix.

The report was penned by then-DOJ Inspector General Michael Horowitz, who currently holds the same position at the Federal Reserve Board and Consumer Financial Protection Bureau.

It is unclear whether the FBI has since carried out a more thorough probe of the hard drives since the 2018 watchdog report was released.

During the 2016 campaign cycle, the FBI Cyber Division had sought to obtain access to the hard drives to conduct targeted searches for information relevant to the Clinton probe — only to be rebuffed, witnesses told the watchdog.

Then-FBI Deputy Director Andrew McCabe asked then-US Deputy Attorney General Sally Yates in a memo if he could refer to the drives for the bureau’s probe of Russian interference in the 2016 election.

That request was also shot down due to concerns that the FBI’s planned search parameters were too broad and didn’t properly safeguard privileged information.

The thumb drives in question were “queried” at least three times, but the purposes behind each search were redacted. At least one of those queries came from former special counsel Robert Mueller’s team. One witness searched for Clinton’s name and found a redacted number of results, including “clintonemail.com.”

Grassley faulted former FBI Director James Comey for the bureau’s failure to “perform fundamental investigative work,” saying investigators “left key pieces of evidence on the cutting room floor.”

“The Comey FBI’s negligent approach and perhaps intentional lack of effort in the Clinton investigation is a stark contrast to its full-throated investigation of the Trump-Russia collusion hoax, which was based on the uncorroborated and now discredited Steele dossier,” claimed Grassley, 91.

“Comey’s decision-making process smacks of political infection.”

Another notable detail in the appendix is that the FBI stumbled upon Russian-language intelligence alleging that then-US Attorney General Loretta Lynch and Comey were interfering in the Clinton email investigation “to help the Democratic and Republican presidential candidates, respectively.”

The intel also included claims that then-Democratic National Committee chairwoman, Rep. Debbie Wasserman Schultz (D-Fla.), had communicated with two individuals who worked at the Soros Open Society Foundations and told them she was confident that the FBI didn’t have evidence against Clinton, because “data was removed from the mail servers just in time.”

Investigators translated the intelligence documents into English, and officials speculated that the intelligence could have come from a cyber attack at the Atlantic Council, a DC-based international affairs think tank.

The documents also claimed that Comey, who registered as a Republican prior to 2016, was set on “dragging this investigation until the presidential elections, in order to effectively undermine the chances” for Clinton to win.

But Peter Strzok — then the FBI deputy assistant counterintelligence director and the lead agent on the investigation — pushed his team to comb through the Russian documents via keyword searches and concluded there were no underlying hacked communications included.

Comey concluded that the documents weren’t credible, and witnesses told the watchdog that they contained information that was “verifiably false.”

The former FBI honcho also defended his decision not to inform Lynch before going public with his July 6, 2016, statement declaring that the feds wouldn’t pursue criminal charges against Clinton over the email debacle, arguing that he was afraid the announcement would leak.

Comey’s statement exonerating Clinton came prior to interviews with multiple key witnesses related to the probe.

The FBI later opened its Crossfire Hurricane investigation into President Trump, looking into whether his campaign colluded with the Kremlin to secure the White House in 2016.

“Today, the Department of Justice honored Chairman Grassley’s request to release information relating to former-FBI Director James Comey’s failed investigation into Hillary Clinton’s mishandling of highly classified information during her tenure as Secretary of State,” Attorney General Pam Bondi said in a statement.

“I commend Chairman Grassley for his unwavering, years-long commitment to exposing the truth and holding those who seek to conceal it accountable. This Department of Justice is fully committed to transparency and will continue to support good-faith efforts in Congress to ensure accountability across the federal government.”