Barr also said he expected to release a redacted version of Mueller's almost 400-page report next week - a slight change from the estimate he gave Tuesday, when he said the release would be within a week.
Barr is appearing before a Senate Appropriations subcommittee one day after he testified before a House subcommittee and answered many - though not all - questions from lawmakers about the release of Mueller's almost 400 page report.
Democrats repeatedly criticized Barr for his handling of the report, including taking it upon himself in the letter to decide that Trump should not be charged with obstruction of justice.
"Even for someone who has done this job before, I would argue it's more suspicious than impressive", Ms Lowey said. When asked, Barr said those details would likely be in the public version of the report.
He has backed away from calls to release the full report and has resumed denouncing the investigation.
He said he had offered Mr Mueller the chance to review his four-page letter, but that Mr Mueller declined.
Barr is facing a different type of high-pressure situation as he determines how much of Special Counsel Robert Mueller's report on Russia's role in the 2016 election and contacts between Moscow and Trump's campaign should be made public. If Congress agrees, there would be 100 new immigration judges, which the attorney general says are necessary to help address a backlog of about 820,000 pending cases nationwide. He said he's open to eventually releasing some of the redacted material after consulting with congressional leaders, but he drew a line at releasing grand jury material, which would require court approval.
She also grilled Barr on how he managed to write his March 24 letter just two days after receiving Mueller's report, which is reportedly 300 to 400 pages.
With the goal to discredit what's coming, Trump and his allies have unleashed a series of broadsides against Mueller's team and the Democrats pushing for full release of the final report.
The chairman of the subcommittee, Democratic Rep. Jose Serrano of NY, also said there were unanswered questions, including "serious concerns about the process by which you formulated your letter; and uncertainty about when we can expect to see the full report". Barr responded that he didn't know what their concern was, but that he suspected they wanted more put out. Barr said he and Deputy Attorney General Rod Rosenstein then concluded that there was insufficient evidence for an obstruction crime.
With or without the threat of impeachment, there is a possibility that Democrats could obtain disclosure if a court, recognizing that the fight over the Mueller materials is an extraordinary circumstance, exercises its authority over grand jury matters to make a disclosure.
But Barr sparred with House Democrats who pressed him on why he would not release grand jury material or provide the full, unredacted Mueller report to Congress.
Attorney General William Barr recently testified before the House Appropriations subcommittee regarding the Department of Justice's 2020 budget. Last week, the House Judiciary Committee authorized a subpoena for the full report in case Barr does not make the entire document available.
"We've done everything we could for the last weeks and weeks to try to reach an accommodation with the attorney general under which we would see the report and the underlying evidence", Nadler said.
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