US Speaker Nancy Pelosi has accused the Attorney General William Barr of lying to Congress.
Her comments came a day after Mr Barr's testimony to a Senate panel about Special Counsel Robert Mueller's report on Russia's alleged meddling in 2016.
Mr Barr faced sharp questioning about his decision to clear President Donald Trump of obstruction of justice.
The top US law official refused to testify to Democratic-led House Judiciary Committee on Thursday.
The stand-off raises the prospect that Mr Barr - America's top legal official - could be held in contempt of Congress.
"He lied to Congress. And if anybody else did that it would be considered a crime," Ms Pelosi said on Thursday.
"Nobody is above the law," she added.
What did Ms Pelosi say?
Ms Pelosi's accusation stems from Mr Barr saying he was not aware of any complaints Mr Mueller had about the attorney general's four-page summary of his report.
Mr Mueller wrote a letter to Mr Barr saying the summary lacked "context".
"It wasn't about technicalities," Ms Pelosi told reporters. "The attorney general of the United States of America was not telling the truth to the Congress of the United States. That's a crime."
But a spokeswoman for the department of justice hit back at Ms Pelosi after her comments.
"The baseless attack on the attorney general is reckless, irresponsible and false," spokeswoman Kerri Kupec said.
Republicans also condemned the remarks. Senator Lindsey Graham told reporters it was an "offensive statement" and said Ms Pelosi should apologise.
"I think it says more about her than it does about Bill Barr," he added.
Can Democrats punish the attorney general?
In a press conference on Thursday morning, Nancy Pelosi directly accused William Barr of committing a crime.
What are Democrats going to do about it? Ms Pelosi only made vague references to a "process", but they have several options if they want to punish the attorney general.
They could make a criminal referral for lying to Congress, which would be forwarded to a US attorney in Mr Barr's justice department. They could hold Mr Barr in contempt of Congress, which would have to be enforced by US attorneys in the Trump administration.
The House of Representatives could also vote to formally censure the attorney general, putting a black mark on his record but not much more.
Finally, they could try to impeach the attorney general and have him removed from office. The process would be similar to that for removing a US president - a majority vote in the House of Representatives followed by a trial in the US Senate requiring a two-thirds vote to "convict".
Only one cabinet official has ever been impeached - Ulysses S Grant's Secretary of War William Belknap in 1876 - and he was ultimately acquitted of corruption charges by the Senate.
What did the attorney general say?
William Barr appeared before the Senate Judiciary Committee on Wednesday and defended his handling of the Special Counsel's 448-page investigation.
It came after Mr Mueller's letter was released, revealing he had twice requested that Mr Barr give more information about the report's conclusions.
But Mr Barr said the special counsel was not concerned with the accuracy of the summary and only with how it was being reported by media.
"My understanding was his concern was not the accuracy of the statement of the findings in my letter, but that he wanted more out there to provide additional context to explain his reasoning and why he didn't reach a decision on obstruction," he said.
Mr Barr also declared he was "absolutely" confident in his own judgment that Mr Trump did not unlawfully attempt to impede the investigation.
But Democratic Senator Mazie Hirono told the attorney general at Wednesday's hearing that he had betrayed a "sacred trust".
"America deserves better. You should resign."
And a tweet showing Democratic Senator Kamala Harris questioning Mr Barr at Wednesday's hearing has now been viewed more than four million times.
Republicans however defended Mr Barr, saying the issue of whether or not President Trump obstructed justice or colluded with the Russians had been settled.
"As far as I'm concerned, this is over," Senator John Kennedy tweeted. "It's time to move on."
The attorney general was scheduled to appear before House Judiciary Committee on Thursday for further questioning about the Mueller probe, but failed to appear.
What have Democrats said?
There has been a growing call for Mr Barr to resign, including from numerous candidates for the Democratic Party's presidential nomination in the 2020 election.
Ms Harris and her colleagues Senators Elizabeth Warren and Kirsten Gillibrand all called for him to step down on Twitter, as did former Texas congressman Beto O'Rourke.
Democratic Committee chair Jerrold Nadler has threatened to hold Mr Barr in contempt of Congress after he missed a deadline to release the full Mueller report.
"We will make one more good-faith attempt to negotiate and to get the access to the report that we need, and then if we don't get that, we will proceed to hold the attorney general in contempt," he told reporters.
In a more light-hearted display, Democratic congressman Steve Cohen brought a bucket of fried chicken and a chicken statue to the House Judiciary Committee hearing on Thursday, and left them in Mr Barr's vacant seat.