WASHINGTON — Donald Trump Jr., former President Donald J. Trump’s eldest son, has agreed to meet soon with the House committee investigating the Jan. 6 attack on the Capitol, according to a person familiar with matter.

The committee has not issued a subpoena for Donald Trump Jr.’s testimony, but he is expected to answer questions voluntarily, the person said. His testimony is slated to come after his fiancée, Kimberly Guilfoyle, met with the panel on Monday for a lengthy interview, which the panel had said would focus on her activities the morning of Jan. 6, 2021, including an Oval Office meeting with the former president, and her role raising money for the rally that preceded the attack on the Capitol.

Donald Trump Jr.’s agreement to testify was reported earlier by ABC News. It was confirmed by a person familiar with it who spoke on the condition of anonymity because the talks about the younger Mr. Trump’s plans to appear were confidential.

He would be the latest family member of the former president to meet with the committee, which is investigating the deadliest attack on the Capitol since the War of 1812. More than 150 police officers were injured in the violence as a pro-Trump mob, believing the former president’s lie of a stolen election, stormed the building. Already, Mr. Trump’s sister Ivanka Trump and her husband, Jared Kushner, have testified during lengthy sessions with the panel.

The cooperation of members of Mr. Trump’s family stands in contrast to the stance taken by some allies of the former president, who have refused to meet with the committee or turn over documents. The panel has not decided whether to ask the former president for an interview.

The panel’s investigators have learned about some of the younger Mr. Trump’s actions concerning the effort to overturn the 2020 election through text messages he sent to Mark Meadows, the White House chief of staff at the time.

Donald Trump Jr. sent one message two days after Election Day in 2020 that laid out strategies for declaring his father the winner regardless of the electoral outcome.

“We have multiple paths,” he wrote to Mr. Meadows on Nov. 5, 2020. “We control them all.”

The message went on to lay out options that Mr. Trump or his allies ultimately employed in trying to overturn the results of the election, including legal challenges, promoting alternative slates of electors and focusing efforts on the statutory date of Jan. 6 for Congress’s official count of the Electoral College results.

A lawyer for Donald Trump Jr. has said the message “likely originated from someone else and was forwarded.”

The younger Mr. Trump also texted with Mr. Meadows during the riot, urging him to move the president to act as the violence played out.

In December, Representative Liz Cheney, a Wyoming Republican and the vice chairwoman of the committee, read aloud at a public meeting from text messages Donald Trump Jr. sent to Mr. Meadows amid the Capitol siege, urging the president to speak out against the mob violence.

“He’s got to condemn this shit ASAP,” the younger Mr. Trump texted Mr. Meadows.

“I’m pushing it hard,” Mr. Meadows responded. “I agree.”

In another message, the younger Mr. Trump implored Mr. Meadows: “We need an Oval address. He has to lead now. It has gone too far and gotten out of hand.”