A senior Argentinian official has accused the US senator Marco Rubio of “imperialist behaviour” after he expressed doubts about the government’s ability to conduct a credible investigation into the mysterious death of a prominent local prosecutor.

Cabinet chief Jorge Capitanich warned the Florida Republican against “unwarranted meddling” in a case that has outraged many Argentinians and put heavy pressure on President Cristina Fernández de Kirchner to determine what happened.

Alberto Nisman, who was investigating a 1994 bombing that killed 85 people at a Jewish community centre in Buenos Aires, was found on 18 January sprawled on his bathroom floor in a pool of blood, a single bullet to his head.

While preliminary autopsy results suggested he killed himself, many Argentinians remain sceptical. Fernández has suggested she believes Nisman was killed by rogue spies but has not offered further details.

Days before his death, Nisman claimed Fernández tried to whitewash his investigation and said he had further evidence to present to Congress.

On Thursday, Rubio sent a letter to John Kerry, US secretary of state, that said: “I am deeply concerned about the ability of the government of Argentina to conduct a fair and impartial investigation into [Nisman’s] death.”

Rubio, a potential Republican candidate for president in 2016, urged Kerry to push for an independent, internationally assisted investigation.

Asked about Rubio’s comments at his daily news briefing, Capitanich said “unwarranted meddling by a state in the internal affairs of another is an interference that is wrong.”

Nisman claimed Fernández had opened secret channels with Iran to clear a number of Iranian suspects in the bombing. Fernández’s government branded the claim “absurd.“

Iran has vigorously denied any role in the bombing.

In the letter to Kerry, Rubio wrote that Nisman’s case had a bearing on US national security. “As you intensify discussions with the Iranian regime … it is more important than ever for the American public and lawmakers to clearly understand the nature of Iran’s activities in our own hemisphere, now as well as in the past.”

