WASHINGTON (AP) — Former Deputy Attorney General Rod Rosenstein told lawmakers Wednesday that he would not have approved an FBI surveillance application for a former Trump campaign aide during the Russia investigation had he known at the time about the problems that have since been revealed.

Rosenstein’s comments amounted to a striking concession that law enforcement officials made mistakes as they scrutinized ties between Russia and Donald Trump’s 2016 presidential campaign. But even as he acknowledged the legitimacy of anger from Trump and his allies, he defended his appointment of special counsel Robert Mueller to lead the probe and affirmed his support for the conclusion that Russia interfered but did not criminally conspire with associates of the Trump campaign.

“I do not consider the investigation to be corrupt, senator, but I certainly understand the president’s frustration given the outcome, which was in fact that there was no evidence of conspiracy between Trump campaign advisers and Russians,” Rosenstein said.

His appearance before the Senate Judiciary Committee was for the first in a series of hearings scrutinizing the FBI’s Russia investigation and the law enforcement officials involved. With subpoena authority expected to be approved this week, the hearing marked the opening salvo of the GOP’s election-year congressional investigation into what they say are damaging findings about the Russia probe from an inspector general review.

The president’s allies have taken fresh aim at the Russia investigation over the last year, pointing to newly declassified information to allege that Trump and his associates were unfairly pursued. They have also claimed vindication from the Justice Department’s decision to dismiss the case against ex-national security adviser Michael Flynn while at times advancing unsupported theories against Obama administration officials.

“We’re going to look backward so we can move forward,” committee chairman Sen. Lindsey Graham, R-S.C., said in explaining the purpose for the hearings. “If you don’t like Trump, fine, but this is not about liking Trump or not liking Trump. This is about us a nation.”

Graham also questioned whether Mueller should have been appointed at all. Rosenstein, who appointed Mueller in May 2017, said he believed there had been a sufficient basis for the investigation and for the naming of a special counsel. But when Graham asked if he would agree with the general statement that by August 2017, there was “no there there” when it came to a criminal conspiracy between Russia and Trump, Rosenstein said yes.

Democrats lamented the hearing’s politically charged and retrospective nature, saying Republicans were attempting to refocus attention away from more urgent problems, including unrest in cities set off by the death of George Floyd and the coronavirus pandemic.

“This hearing wastes this committee’s time in a blatant effort to support the president’s conspiracy theories and to help the president’s reelection,” said Democratic Sen. Mazie Hirono of Hawaii.

The hearing delved into detail in two areas that Trump allies have recently seized on to challenge the conduct of law enforcement.