• Robert F. Kennedy Jr., an independent candidate for president, is being questioned by the Federal Election Commission for payments his campaign made to his daughter-in-law last year.
• The FEC sent Kennedy's campaign a letter on Tuesday, asking for details about "payments to members of the candidate's family" between July 1 and Sept. 30.
• The FEC threatened to take legal action against the Kennedy campaign if the payments to family were for personal use and not campaign services.

Robert F. Kennedy Jr., an independent candidate for president, is being questioned by the Federal Election Commission for payments his campaign made to his daughter-in-law last year, according to a new letter first reviewed by CNBC.

The FEC sent Kennedy's campaign a letter on Tuesday, asking for details about "payments to members of the candidate's family" between July 1 and Sept. 30.

The FEC threatened to take legal action against the Kennedy campaign if the payments to family members did not reflect the fair market value of legitimate campaign services.

Although the letter doesn't name the family member, Kennedy's daughter-in-law, Amaryllis Fox Kennedy, is the only family member who received payments from Robert F. Kennedy Jr.'s campaign for president during the period in question.

Robert F. Kennedy Jr., a son of the late Attorney General Robert F. Kennedy, rose to fresh prominence in recent years as a vaccine skeptic. A lifelong Democrat, Kennedy launched a campaign for president against fellow Democrat Joe Biden last April. In October, he switched to a third-party run.

Polling on Kennedy has varied widely, and it's unclear how serious a threat he poses to Biden's reelection effort. In December, Quinnipiac University released a survey of registered voters that showed Kennedy would win 22% of the vote in a hypothetical three-way race, with Trump at 36% and Biden taking 38%.

Amaryllis Fox Kennedy is a former Central Intelligence Agency officer who is married to Robert F. Kennedy Jr.'s son, Robert F. Kennedy III.

Despite lacking any obvious experience in political campaigns, Kennedy was officially named manager of her father-in-law's presidential campaign in October. She replaced former Rep. Dennis Kucinich, D-Ohio, the previous campaign manager.

Until her father-in-law ran for president, Amaryllis Fox Kennedy had never received compensation from a federal political campaign, according to FEC records.

Beginning in June of last year, FEC records show that Kennedy's daughter-in-law began collecting a salary from the campaign for what it described in filings as "administrative" work.

Over the next four months, the campaign reported paying Amaryllis Fox Kennedy approximately $70,000.

"Salary payments made to members of the candidate's family constitute personal use of campaign funds unless the family member is providing bona fide services to the campaign," the FEC said in its letter to the Kennedy campaign on Tuesday.

"If a family member is providing bona fide services to the campaign, any salary payment in excess of the fair market value of the services provided is personal use," the commission wrote.

A Kennedy campaign spokesperson did not respond to CNBC's request for comment on the letter. The campaign has until Feb. 13 to respond to the FEC.

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