Top Mueller deputy reveals why he chickened out of forcing Donald Trump Jr. to testify

One of the mysteries left in the wake of special counsel Robert Mueller's investigation was why Donald Trump Jr., who infamously met with Russian agents to discuss the hacking of Hillary Clinton's stolen emails, was never forced to testify.

Andrew Weissmann, who served as one of Mueller's top deputies during the probe, has now given an answer.

In an excerpt of Weismann's upcoming book on the investigation obtained by The Atlantic, Weissmann claims that Mueller was too worried about the president shutting down the entire probe if they subpoenaed his family members to testify.

"The specter of our being shut down exerted a kind of destabilizing pull on our decision-making process," Weissmann writes.

Mueller was supposedly especially fearful of bringing in first daughter Ivanka Trump, who was never even asked to testify, let alone slapped with a subpoena.

"[They] feared that hauling her in for an interview would play badly to the already antagonistic right-wing press -- Look how they’re roughing up the president’s daughter -- and risk enraging Trump, provoking him to shut down the Special Counsel’s Office once and for all," Weissmann explains.

Weissmann lays much of the blame for this overly cautious approach at the feet of attorney Aaron Zebley, another top Mueller deputy who always argued against taking aggressive actions.

"Repeatedly during our twenty-two months in operation, we would reach some critical juncture in our investigation only to have Aaron say that we could not take a particular action because it risked aggravating the president beyond some undefined breaking point," he writes.

Weismann’s book — titled “Where Law Ends: Inside the Mueller Investigation” — is due to be released on September 29th.