Sen. Tim Kaine, D-Virginia, suggested Saturday that some officials at the FBI are "actively working" to support Republican presidential candidate Donald Trump.

Kaine, the Democratic vice-presidential candidate, made the claim during an interview with Fusion, a Miami-based television network targeting bicultural millennials. The comments mark an escalation in the Clinton campaign's response to the FBI's renewed inquiry into Hillary Clinton's use of a private email server when she served as secretary of state.

Kaine called the FBI a "leaky sieve" and criticized director James Comey for breaking agency protocol by discussing a politically sensitive case so close to an election. He also dismissed former New York mayor Rudolph Giuliani's decision to back off claims that he was given advance notice of the FBI's plans to possibly reopen the Clinton investigation.

"I don't think Giuliani's walk-back is credible," Kaine said. "I think the FBI sadly has become like a leaky sieve."

Giuliani, a prominent Trump supporter, told Fox News in the days before Comey alerted lawmakers to the FBI's renewed inquiry that there would be a "big surprise" coming from the agency. Asked about his claims again on Friday, Giuliani told Fox's "Fox and Friends" program, "you're darn right I heard something." But Giuliani pulled back on Saturday, saying he was only aware of "tremendous anger" from former FBI agents upset with Comey's decision.

Comey's decision to alert Congress about his review of the Clinton case "suggests that it's probably more likely explained that [Comey] knew that the FBI is not only a leaky sieve but there were people within the FBI actively working - actively working - to try to help the Trump campaign," Kaine said. "This just absolutely staggering, and it is a massive blow to the integrity of [the FBI]."

Kaine added he thinks that Comey was under pressure to release information to Congress because "subordinates would do it if he didn't."

The FBI decided to review the Clinton case after discovering new emails potentially relevant to the original investigation. Senior FBI officials were informed about the discovery of new emails, obtained in relation to an investigation of former congressman Anthony Weiner, D-New York, at least two weeks before Comey notified Congress, federal officials familiar with the investigation have told The Washington Post.

The officials said that Comey was told that there were new emails before he received a formal briefing and opted to inform lawmakers.

In the wake of Comey's announcement, the Clinton campaign has publicly questioned Comey's motives and fitness to serve - despite praising his leadership of the FBI after he announced in July that the agency wouldn't recommend the email case for prosecution. Clinton herself has raised the issue on the stump, while top aides and surrogates have called Comey's judgment into question.

Kaine was campaigning on Saturday in Florida when he made his remarks to Fusion. He is scheduled to maintain a breakneck pace in the closing hours of the campaign, including an appearance Sunday on CBS's "Face the Nation" -- where he'll likely be asked to clarify his comments to Fusion - and a visit to Wisconsin. On Monday he will campaign in his home state, as well as Pennsylvania and North Carolina.