Attorney General Bill Barr told a Senate Appropriations subcommittee on Wednesday that he believes spying by law enforcement officials on the 2016 Trump campaign "did occur," before clarifying at the end of the hearing: "I am not saying that improper surveillance occurred. I am saying that I am concerned about it and I’m looking into it."

The exchange:

Barr: "For the same reason we're worried about foreign influence in elections, we want to make sure that — I think spying on a political campaign is a big deal. ... There were a lot of rules put in place to make sure that there's an adequate basis before our law enforcement agencies get involved in political surveillance. I'm not suggesting those rules were violated, but I think it's important to look at that."

"For the same reason we're worried about foreign influence in elections, we want to make sure that — I think spying on a political campaign is a big deal. ... There were a lot of rules put in place to make sure that there's an adequate basis before our law enforcement agencies get involved in political surveillance. I'm not suggesting those rules were violated, but I think it's important to look at that." Sen. Jeanne Shaheen (D-N.H.) : "But you're not suggesting that spying occurred?"

: "But you're not suggesting that spying occurred?" Barr: "I don't ... well ... I guess you could — I think spying did occur. Yes, I think spying did occur. But the question is whether it was predicated, adequately predicated. And I'm not suggesting that it wasn't adequately predicated. But I need to explore that."

When pressed by Sen. Jack Reed (D-R.I.) whether he has any evidence of wrongdoing by the FBI or Mueller in the Russia investigation, Barr said: "I have no specific evidence that I would cite right now. I do have questions about it." Barr said he plans on reviewing "both the genesis and conduct" of the Russia investigation.

Why it matters: Defenders of President Trump have long accused Obama-era intelligence officials of spying on the Trump campaign for political reasons, alleging that intelligence tools, like the FISA process, were abused. These claims have not been corroborated.

A redacted FISA warrant for Carter Page, a former Trump campaign adviser, was released by the Department of Justice last year.

The documents appeared to show that the FBI properly disclosed its sources of information and that it relied on more than just the controversial Steele dossier, contradicting claims of abuse by Republicans.

Go deeper: How FISA works