WASHINGTON – Attorney General William Barr told a Senate panel Wednesday that he is reviewing whether federal authorities improperly spied on President Donald Trump's campaign during the early stages of the investigation into whether any of his aides participated in Russian efforts to sway the 2016 election in Trump's favor.

"Spying on a campaign is a big deal," Barr told a Senate panel Wednesday. "I think spying did occur. The question is whether it was adequately predicated."

The attorney general said he planned to examine the "genesis and the conduct" of the FBI's investigation into possible ties between the Trump campaign and Russia, launched in the midst of the 2016 presidential run. The inquiry was ultimately turned over to special counsel Robert Mueller to complete, and Barr has said it concluded that Trump's campaign did not conspire with the Russian government.

"I am not saying that improper surveillance occurred," Barr told the Senate Appropriations subcommittee. "I am concerned about it. There is a basis for my concern."

Barr, however, declined to say what specific concern prompted his review or to describe the parts of the investigation he considered to be spying.

Earlier Wednesday, Trump characterized the Mueller investigation as both "illegal" and an "attempted coup," while expressing support for a review by Barr.

"There is a hunger for this to happen," Trump told reporters.

The department's inspector general is conducting a review of surveillance warrants authorities used to eavesdrop on a former campaign aide, Carter Page, in October 2016. Barr has said that effort should be completed by June. Republicans in Congress have complained repeatedly that the FBI targeted Trump's campaign for political reasons, revealing text messages between two senior officials involved in the probe who expressed their personal contempt for Trump.

So far, however, investigations in Congress have not produced evidence that the extraordinary inquiry into whether Trump's campaign cooperated with Russian efforts to sway the 2016 election that put him in office were politically motivated or meant to sabotage his campaign.

Among the questions Barr said he would seek to answer is why the Trump campaign was not notified in advance by federal authorities of its examination of Russian election interference.

"I don't understand why the campaign was not advised," Barr said.

Building on his testimony a day earlier before a House committee, Barr told lawmakers Wednesday that he planned to ask other Justice Department officials to examine whether the department abused its surveillance powers. He said he had not formed a view of whether the department or other intelligence agencies acted improperly, but the sensitivity of using national security tools to investigate the conduct of a political campaign required scrutiny.

Barr said that he did not view surveillance abuse as "a problem endemic to the FBI," but suggested that there was likely "a failure among a group of leaders there."

The attorney general's references to possible surveillance abuses came at a crucial time, as Barr prepared to release a redacted version of Mueller's final report on the Russia investigation "next week."

Republicans immediately seized on the statements, seeking to use them to cast the government's investigation of Russia and the Trump campaign as illegitimate. "Now, the investigation has flipped," said Brad Parscale, manager of Trump's re-election campaign. "This attempted coup needs to be fully investigated."

Democrats said Barr's choice of words suggested he was courting the favor of a president who has frequently attacked the investigation and the people who conducted it. House Speaker Nancy Pelosi, D-Calif., said Mueller's assessment of the investigation – not Barr's – should hold sway.

"I don't trust Barr; I trust Mueller," Pelosi said in an interview with the Associated Press.

Barr, meanwhile, asserted that some parts of Mueller's report must remain secret because they contain grand jury information, sensitive national security material, may interfere with ongoing inquiries or contain information that could be damaging to the reputations of those who were not charged in the special counsel's investigation.

The attorney general said Mueller and his staff were working to help remove sensitive information from the report so it could be released to Congress and the public. And he defended the summary conclusions he delivered to Congress last month and the speed with which those conclusions were made public.

But lawmakers closely questioned Barr on his decision to release a bare-bones summary of the report last month in which the attorney general said Mueller had not found a conspiracy involving the Trump campaign and the Russian government.

The special counsel did not make a determination about whether the president's actions during the investigation amounted to obstruction. Instead, Barr and his deputy, Rod Rosenstein, separately determined that Trump's conduct did not constitute a crime.

Barr told the Senate on Wednesday that Mueller did not indicate whether he expected Congress to settle the issue of obstruction. Mueller also did not indicate that the decision should be ceded to the attorney general, Barr said. The attorney general added that he and Rosenstein – along with other Justice officials – concluded independently that Trump's conduct was not criminal.

"I don't know whether Bob Mueller supported my conclusion," Barr said.

The attorney general's intervention at the end of Mueller's investigation has set off a politically charged battle for access to the entire text of the special counsel's report, the fruit of a 22-month investigation that has shadowed the Trump administration since its first days.

While Barr had been called before the Senate and House committees this week to discuss the Justice Department's $29 billion budget, the focus of both hearings was trained on his handling of Mueller's report and when the document would be made public.

On Tuesday, Barr told the House committee that he had no plans to ask for a court's permission to disclose grand jury material but said he would consult with Congress after he releases the report to determine whether he can provide additional information.

House Democrats vowed last week that they would seek Mueller's testimony in addition to that of Barr, who also is expected to address the matter for congressional committees in early May.

Justice, however, has defended Barr's handling of the report, suggesting that the full document was so packed with secret grand jury information that revealing even portions of it immediately would have been impossible.

Contributing: David Jackson