Turkish authorities have said they believe Khashoggi, a columnist for The Washington Post, was killed inside the consulate, a startling allegation that is firmly denied by the Saudis.
The series of tweets that Trump Jr. retweeted Friday included a post by Patrick Poole, who according to his Twitter profile is a national security and terrorism correspondent for PJ Media, a conservative-leaning collaborative news and commentary blog formerly known as Pajamas Media.
In his tweet, Poole shared images of a decades-old newspaper article written by Khashoggi about the mujahedeen.
In a photo accompanying the article, Khashoggi, a prominent journalist in the Middle East, appears beside individuals identified as members of the Islamist group and holds a rocket-propelled grenade launcher. A separate photograph in the article pictures Osama bin Laden and some of his associates. Bin Laden cofounded al Qaeda in the late 1980s.
Khashoggi covered al Qaeda as a foreign correspondent in Afghanistan and Sudan and was well known for his several interviews with Osama bin Laden. He distanced himself from bin Laden after the September 11, 2001, terrorist attacks. At the time of the news article posted by Poole, the United States was arming and funding the mujahedeen in the Soviet-Afghan War.
"Huh. It's almost like reality is quite different than the evidence-free narratives peddled by media with a long history of cooperating with or getting duped by Iran echo chamber architects," Davis wrote on Twitter.
Trump Jr. then retweeted Davis' comment.
The Trump Organization on Friday night did not immediately return CNN's request for comment on Trump Jr.'s Twitter post.
"We are pretty clear eyed it is likely to have happened, and it didn't end well," said the official, who also cautioned that this was the latest assessment and no conclusions had been made.
Although the Saudis have issued a broad denial of responsibility, they have not offered concrete evidence to prove Khashoggi walked out of their consulate or is still alive.
On Friday, Trump vowed to raise the matter with Saudi King Salman and claims that answers will come "sooner than people think," but senior administration officials described a process that could take weeks, if not months, to determine what precisely happened to Khashoggi and develop a US response if it is determined he was killed on orders from the Saudi government.
Trump has made clear that his administration will not make human rights overseas a priority; he and especially his son-in-law and senior adviser, Jared Kushner, maintain close ties to the crown prince; and Trump has long had business ties to Saudi Arabia, where he made his first foreign trip as President.
In addition, Trump has established a trio of goals in the Middle East that depend on Saudi Arabia's rulers and their money. The kingdom is central to the Trump administration's goals on Middle East peace, its effort to fight ISIS in Syria and elsewhere, and its foreign policy priority of countering Iran.
That combination of factors explains low-key US responses following aggressive Saudi foreign policy moves and the muted response to Khashoggi's suspected assassination. They also create a dilemma for the White House, as lawmakers moved Wednesday to force a showdown over US values in foreign policy by triggering a human rights probe that could lead to sanctions.
"There are other ways of punishing, to use a word that's pretty harsh but it's true," he said, adding later, "We're going to get to the bottom of it and there will be severe punishment."
CNN's Kevin Liptak, Bard Wilkinson, Nic Robertson and Laura Smith-Spark contributed to this report.