When Robert F Kennedy Jr ran for president as a Democrat in 2023, he found an unexpected ally in Scott Brown. A former Republican senator, Brown had begun a tradition of hosting Republican presidential candidates for barbecues in his New Hampshire back yard, where they could stump for votes and get attention ahead of the state’s crucial primary.

Kennedy became Brown’s first Democratic invitee. His appearance in September 2023 drew hundreds of people, Brown’s biggest crowd ever. Kennedy held Brown in such high regard that after he decided to run instead as an independent, he reportedly reached out to Brown as a possible vice-presidential running mate, though Brown declined.

Later, Brown – now running for Senate in New Hampshire – said he helped Kennedy prepare for his Senate confirmation hearings for the job of US health secretary.

Newly released records obtained by the Guardian shed further light on the men’s relationship and how Brown – who had been appointed to serve as ambassador to Samoa and New Zealand by Donald Trump during his first term – responded to a controversial trip Kennedy took to Samoa in June 2019. Months later, Kennedy’s visit would become a subject of public scrutiny when measles swept through the Pacific island nation. Samoan officials and health leaders said Kennedy, who made the trip when he was the head of a US anti-vaccine organization, bolstered the credibility of local anti-vaccine activists ahead of the outbreak, which killed 83 people, mostly children under age five.

Kennedy has said he had “nothing to do with people not vaccinating in Samoa” and that he never told anybody there not to vaccinate.

Details about the Samoa trip follow reporting last month by the Guardian and the Associated Press, which obtained emails that undermined Kennedy’s testimony during his Senate confirmation hearings that his visit had “nothing to do with vaccines”. Two Democratic senators and a member of the House said the reporting showed Kennedy lied to the Senate.

In addition, Brown is running in the Republican primary for an open Senate seat in New Hampshire at a time when control of the Senate hinges on which party flips or retains a handful of seats. Vaccine policy has become a hot topic leading into the midterms, as some Republicans are reportedly worried that Kennedy’s endorsement of anti-vaccine views and policy changes could hurt their candidates at the polls.

The Guardian received emails and other records as part of a second release of records by the US Department of State resulting from a lawsuit brought with the assistance of the Reporters Committee for Freedom of the Press.

Kennedy traveled to Samoa at a time when vaccination rates had plummeted. The year before, two babies died when they were injected with a tainted measles, mumps and rubella vaccine that had been improperly prepared. The government halted the vaccine program for months. When Kennedy got there, vaccinations had resumed, but many people still hesitated to get their children immunized.

The emails show that Brown was kept apprised of Kennedy’s trip by a top embassy staffer in Samoa, who alerted the ambassador that Kennedy was motivated to visit because of his views about vaccines. But even as the deputy chief of mission in Samoa, Antone Greubel, took steps to distance the embassy from Kennedy’s trip, emails indicate that Brown met with Kennedy at the US embassy.

It is not clear what Kennedy and Brown discussed, how long they spent with each other or whether they discussed vaccines. Neither Brown’s campaign nor Kennedy’s spokespeople responded to multiple messages seeking comment. Neither did the US state department.

Before joining the first Trump administration, Brown had been a Republican senator from Massachusetts, winning a special election in 2010 after Kennedy’s uncle, the Democratic “Lion of the Senate” Edward Kennedy, died in office. But his time there was short-lived: he lost the seat to Democrat Elizabeth Warren in 2012.

Embassy officials became aware of Kennedy’s trip in May 2019, the records show. Greubel sent an email to Brown on 24 May 2019, with the subject line “Robert Kennedy, Junior May Come to Samoa.” He noted that Kennedy might travel there the following week and if he did, he would probably be a VIP guest at Samoa’s independence day celebration on 1 June.

“The real reason Kennedy is coming is to raise awareness about vaccinations, more specifically some of the health concerns associated with vaccinating (from his point of view),” Greubel wrote to Brown. He also told Brown at that time that US embassy staffer Benjamin Harding had been involved in a personal capacity in arranging the visit and that Greubel had told Harding to “cease and desist from any further involvement with this travel”, though the rest of the sentence is redacted.

Brown replied the same day, asking Greubel to keep him posted.

Meanwhile, emails show Greubel gathering information about Kennedy’s trip, and exchanging emails with a human resources officer about Harding.

Four days after his initial email, on 28 May 2019, Greubel followed up with his colleagues in New Zealand.

In a volley of emails about Kennedy’s impending visit that was labeled as a sensitive human resources matter and heavily redacted, Greubel shared details about how Kennedy’s trip was arranged. The unidentified staffer he wrote to replied, “I forgot he was traveling down here so was surprised to run in to him this morning! This didn’t come up,” ending the line with a smiley face emoji.

Later that day, Greubel followed up in a lengthy email to Brown, the substance of which was entirely redacted by the state department. Brown soon replied: “Thanks. He is here now. Saw him at Embassy.”

Other emails showed Greubel trying to distance the embassy from Kennedy. During Kennedy’s time in Samoa, for example, he received an email from someone with a Gmail address, saying they were “pink with envy” because the person had worked for Kennedy’s father, the late senator Robert F Kennedy, and was at the Ambassador hotel the night he was killed. The person asked if Greubel got to spend time with Kennedy and his wife, Curb Your Enthusiasm actor Cheryl Hines.

Greubel replied: “The US Embassy had nothing to do with RFK, Jr.’s visit to Samoa. He is set to leave today. I talked to him briefly after the Independence Day-eve Church Service and I was in the same room as him for a dinner and lunch.”

Months later, measles hit. By November, the numbers of infections and deaths were climbing.

Records showed the embassy fielded inquiries by media outlets including from New Zealand television station TVNZ and the New York Times about Kennedy’s visit, his meeting with an anti-vaccine blogger and Harding’s involvement in introducing them.

One person who identified themself as an American healthcare professional sent a message to the US embassy in Samoa’s Facebook account that November, which an employee then shared among staff in both Samoa and New Zealand.

“Is it true that the US Ambassador facilitated anti vax activities by RFK, Jr. and others during the summer, despite Samoa’s well-known abysmal vaccination rates and inadequate hospital facilities?” the person, who was not named in the email, wrote.

No response was included in the records so far released to the Guardian.

Five years later, Trump won the 2024 presidential election after forming an alliance with Kennedy, and picked him to run the nation’s Department of Health and Human Services. Kennedy’s nomination drew intense scrutiny.

Brown has said he helped Kennedy get ready to go before the Senate.

“I helped prepare him for his hearings a little bit on certain things that I understood and certain people,” Brown told New Hampshire Today in September, according to a recording posted online by American Bridge, a Democratic political research firm.

During the hearings, Kennedy was asked several times about what happened in Samoa. He noted that vaccination rates were low when he got there, and the outbreak started a few months after he left, “so clearly I had nothing to do with measles”.

He also denied that his purpose in going there had anything to do with vaccines, and he downplayed the severity of the outbreak. He falsely claimed without evidence that most of the 83 people who died did not have measles and “we don’t know what was killing them”. A Samoan health official told the AP his comment was “a total fabrication”. The Hawaii governor, Josh Green, a doctor who responded to the outbreak, called Kennedy a liar.

Neither Brown nor spokespeople at Kennedy’s health department responded to questions about whether the Samoa trip was among the subjects Brown helped Kennedy prepare for ahead of those hearings.