After viewing the results of the FBI's supplemental background check into Supreme Court nominee Brett Kavanaugh on Thursday, a top Democrat called the agency's effort a 'bull---- investigation' and claimed the White House had improperly interfered with the process.

"Well, that report -- if that's an investigation, it's a bull---- investigation," Sen. Bob Menendez, D-N.J, told a man as he walked past. "The reality is, that was not a full and thorough investigaton."

He continued: "Evidently, the Republicans who gave the direction to the FBI of what could be investigated was extremely limited. You know, I hear a lot about a lack of corroboration; well, you don't get corroboration if you don't talk to corroborating witnesses at the end of the day."

Each of the witnesses that Christine Blasey Ford, the California professor claiming Kavanaugh attacked her in the 1980s, identified as present during the alleged attack has said under penalty of felony they do not recall the events she describes. But an attorney for another accuser, Deborah Ramirez, who has said she believes Kavanaugh exposed himself to her at a college party, charged this week that the FBI had not contacted dozens of potential witnesses he pointed them towards.

President Trump has said the FBI had authority to interview "whoever they deem appropriate, at their discretion." As part of a deal brokered last Friday between Sen. Jeff Flake, R-Ariz, and top Democrats, the White House authorized the agency to look into any credible sexual misconduct allegations against Kavanaugh for a maximum of one week.

The time limit, Flake and other Republicans said, was necessary to avoid bogging down Kavanaugh's nomination with a never-ending probe into the accusations, which all related to alleged events more than three decades ago. None of the claims against Kavanaugh has first-hand corroboration, and the credibility of some of his accusers has come under question in recent days.

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"I'm amazed that, for the highest court of the land, this is the kind of report the FBI produces," Menendez said.

While the FBI's investigation reportedly involved extensive interviews with numerous individuals, including Kavanaugh friend Mark Judge, Democrats complained that agents did not interview Ford and other potential witnesses. Ford spoke at length under oath last Thursday before the Senate Judiciary Committee.

Despite Menendez's harsh assessment, the prospects for Kavanaugh's confirmation now rest on four swing senators, as GOP leaders close ranks behind the embattled judge and one undecided Democrat comes out in opposition.

North Dakota Democratic Sen. Heidi Heitkamp, after staying on the fence for weeks, announced Thursday she will vote against Kavanaugh, citing in part his conduct at last week's fiery Hill hearing.

But at least two Republican swing senators indicated they were satisfied with the FBI’s investigation, stirring speculation they could back the embattled judge in the end.

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Sen. Susan Collins, R-Maine, who has stayed mum on her Kavanaugh stance, said Thursday that the bureau’s supplemental background probe “appears to be a very thorough investigation.”

And Flake, who originally requested the FBI re-open its investigation into the claims leveled against Kavanaugh by Christine Blasey Ford, agreed.

“No new corroborative information came out of it,” Flake said. “Thus far, we’ve see no new credible corroboration—no new corroboration at all.”

Other senators were continuing to view the confidential, printed-out FBI report in a secure area of the Capitol.

Fox News' Brooke Singman contributed to this report.