House Speaker Nancy Pelosi is asserting that Attorney General Robert Barr has committed a crime by lying to Congress.

At a news conference Thursday, Pelosi ramped up pressure on Barr, who boycotted a House hearing but testified for several hours Wednesday to a Senate panel about special counsel Robert Mueller’s report.

Pelosi said Barr’s testimony to Congress raised “deadly serious” questions. She said, “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.”

Pelosi didn’t say what testimony she was referring to, but Democrats have seized on Barr’s testimony from last month in which he appeared unaware of concerns from Mueller’s team over the way he had characterized their work.

Barr said Wednesday his answer was not misleading because he had been in touch with Mueller, rather than members of his team, and that the concerns were mostly about process and not substance.

Pelosi also said that administration refusals to respect subpoenas by House committee is “very, very serious” and noted that ignoring congressional subpoenas was one of the articles of impeachment against former President Richard Nixon.

WATCH: William Barr testifies on Mueller report to Senate committee