"In choosing his personal staff, the President enjoys an unusual degree of freedom, which Congress found suitable to the demands of his office," wrote Daniel Koffsky, deputy assistant attorney general in the DOJ Office of Legal Counsel, which serves as interpreter of federal law for the White House.
In essence, Koffsky reasoned that the anti-nepotism law covers only appointments in an "executive" agency and that the White House Office is not an executive agency within the law. He cited a separate law that gives the President broad powers to hire his staff.
That law authorizes the president to appoint "employees in the White House office without regard to any other provision of law regulating the employment or compensation of persons in the government service."
The argument is in line with the interpretation put forward by Kushner's attorney, Jamie S. Gorelick, earlier in the week.
"The appointment of Jared Kushner, President-elect Donald Trump's son-in-law, as a White House senior adviser is clearly lawful under this authority," Gorelick wrote in The New York Times earlier this week.
A person familiar with the issue told CNN that Trump's transition team asked career lawyers with the DOJ Office of Legal Counsel to review the proposed Kushner appointment.
Another source familiar with the matter said Koffsky is a longtime career attorney at OLC. The person familiar with the issue said political appointees of the DOJ, including Attorney General Loretta Lynch, did not play any role in the opinion.
A transition source told CNN earlier this month that Kushner, who is married to the President's daughter Ivanka, will be senior adviser to the president.
Kushner, a 36-year-old Harvard-educated businessman, was a key political strategist on Trump's election. But federal anti-nepotism laws brought into question whether he could continue his influence in the President's life post-election.
Trump -- along with many of his Cabinet nominees -- has repeatedly been criticized for conflict of interest. But the Justice Department downplayed those concerns in this specific hire.
"A President wanting a relative's advice on governmental matters therefore has a choice: to seek that advice on an unofficial, ad hoc basis without conferring the status and imposing the responsibilities that accompany formal White House positions; or to appoint his relative to the White House under title 3 and subject him to substantial restrictions against conflicts of interest," Koffsky wrote.
"We believe that the President's special hiring authority in 3 U.S.C. § 105(a) permits him to make appointments to the White House Office that the anti-nepotism statute might otherwise forbid," Koffsky wrote.
He acknowledged that while his conclusion "departs" from "prior work" of the Office of Legal Counsel, "this departure is fully justified."
In a statement, Gorelick said, "We believed that we had the better argument on this. The Office of Legal Counsel of the Justice Department -- in an opinion by a highly regarded career Deputy Assistant Attorney General -- adopted a position consistent with our own."
This story has been updated.