WASHINGTON — The Justice Department should require high-level approval before the FBI opens an investigation into a major political campaign, the department's inspector general testified Wednesday as he briefed senators on his report into the probe of the 2016 Trump campaign.

In his opening statement to the Senate Judiciary Committee, Inspector General Michael Horowitz expressed misgivings about the FBI's errors and omissions in its requests for judicial approval to conduct surveillance on Trump campaign aide Carter Page.

"We are deeply concerned that so many basic and fundamental errors were made by three separate, hand-picked investigative teams on one of the most sensitive FBI investigations after the matter had been briefed to the highest levels within the FBI," Horowitz said.

Under questioning from Committee Chairman Sen. Lindsey Graham, R-S.C., Horowitz discussed a litany of embarrassing slip-ups by the FBI, including the actions of a lawyer who the inspector general said "doctored" an email to make it seem like Page was not a CIA source, when in fact he was. That potentially exculpatory information was never shared with the Foreign Intelligence Surveillance Court, which approved the surveillance warrant.

Horowitz also said he could not rule out political bias as a possible motivation for the 17 errors the FBI made in applications for the Page surveillance.

"Can you say it wasn’t because of political bias?" Graham asked.

"I do not know," Horowitz replied.

Graham asked Horowitz about an op-ed by former FBI James Comey, who proclaimed that the IG report vindicated him.

"You know, I think the activities we found here don't vindicate anybody who touched this," Horowitz replied.

The ranking Democrat on the committee, Sen. Pat Leahy of Vermont, asked Horowitz about a statement after the release of the IG report by John Durham, the U.S. attorney in Connecticut who has been tasked by Attorney General William Barr with conducting a broader investigation into the origins of the Russia probe.

"Based on the evidence collected to date, and while our investigation is ongoing, last month we advised the inspector general that we do not agree with some of the report’s conclusions as to predication and how the FBI case was opened," Durham said on Monday.

Horowitz told Leahy he was surprised by Durham's statement, and he painted the disagreement as a narrow one. Horowitz said Durham agreed that opening a preliminary investigation was justified, but not a full field investigation, as the FBI opened. That is a narrow and technical difference. A Justice Department spokeswomen was not immediately available for comment.

Through a spokesperson, Durham declined to comment.

Horowitz also said that he found no evidence the FBI sought to insert informants into the Trump campaign. But according to the report he released Monday, the FBI did use "confidential human sources" (CHSs) to speak to and record members of the Trump campaign, including George Papadopoulos, Page and an unnamed senior official.

"There is no applicable Department or FBI policy requiring the FBI to notify Department officials of a decision to task CHSs to consensually monitor conversations with members of a presidential campaign," Horowitz told the Senate Judiciary Committee.

Graham's 'massive conspiracy'

Before the inspector general testified, Graham painted the IG's report in apocalyptic terms as he railed against what he views as an illegitimate investigation into his political ally, President Donald Trump.

He said Horowitz uncovered "a massive conspiracy over time to defraud the FISA court, illegally surveil an American citizen," and keep an investigation going into a sitting president.

Graham added, "What happened here can never happen again. ... People at the highest level of our government took the law into their own hands."

He painted a picture of a group of anti-Trumpers within the FBI who cooked up a bogus investigation.

In fact, the IG found no evidence that political bias tainted the FBI's Russia probe, which was run mostly by career civil servants. Two FBI officials whose texts displayed bias against Trump played a minimal role, the IG report says.

Horowitz did find a series of errors in how the bureau got approval for conducting electronic surveillance of Page, who ultimately was never charged with a crime. And he found some concerns with how the FBI approached its investigation into a presidential campaign.

Horowitz also confirmed that the Office of the Director of National Intelligence and the FBI conducted a counterintelligence briefing for Trump on Aug. 17, 2016, to warn him about foreign threats, including from Russia.

NBC News first reported on this meeting in December 2017.

Trump and his top aides never alerted the FBI to the many approaches by Russians to their campaign, according to former special counsel Robert Mueller's report.

The inspector general report says that the FBI used the August 2016 meeting to test the reaction of Trump adviser Michael Flynn, who by then was under investigation as a suspected foreign agent.

An FBI agent who helped with the briefing also wrote a report, known as an FBI 302.

Sen. Charles Grassley, R-Iowa, said this was an abuse of the briefing process by the FBI. Horowitz did not disagree.

The FBI never conducted a full "defensive briefing" in which it would have warned Trump that members of his campaign were under a counterintelligence investigation. FBI officials told the inspector general that was because they could not be sure who within the campaign was compromised by a foreign power.

In the end, they did not determine that anyone was, although Flynn and Papadopoulos were convicted of lying, deputy campaign chairman Rick Gates pleaded guilty to lying, and campaign chairman Paul Manafort was convicted of fraud.