Secret Service officials, speaking on the condition of anonymity, disputed her account.

But the officials did say Mr. Engel, Mr. Ornato and the driver of the Suburban are prepared to confirm to the committee another damning finding from Ms. Hutchinson’s testimony: that Mr. Trump demanded his agents bring him to the Capitol so he could join his supporters, even after they emphasized the dangerous scene playing out there.

The willingness of the agents to provide potentially critical details about the person they were protecting marks a rare turn for an agency that has historically prioritized the secrecy of presidents, even in the face of investigations…

The dispute also highlights Mr. Trump’s relationship with his Secret Service detail, which was unlike that of most previous presidents. Agents were seen as more overtly supportive and admiring of Mr. Trump than they had been under any other modern president, according to people who have spent time in the White House during multiple administrations, and Mr. Trump worked to build loyalty among them.