Kazem Rajavi, a prominent Iranian academic and opposition figure, was shot dead in his car in the village of Coppet, near Geneva, on April 24, 1990.
The Swiss soon voiced suspicions that Tehran authorities ordered the killing, and drafted a list of 13 Iranians suspected of actively taking part.
And in 2006, the Alpine nation issued an international arrest warrant for Iran's former intelligence minister Ali Fallahian over suspicion he ordered the murder.
But in a letter sent to the lawyer of Rajavi's family, dated May 29 and seen by AFP, the prosecution office in the western Swiss canton of Vaud, where Coppet is located, announced that the investigation would soon be dropped and the case closed.
The letter, signed by prosecutor Augustin Dayer, said the statute of limitation in the case had expired, and asked that any petitions or comments in the matter be filed before June 17.
When reached by AFP on Wednesday, Vaud Prosecutor General Eric Cottier did not comment specifically on the case, but pointed out that the maximum statute of limitation in Switzerland, except in the most extraordinary cases like genocide, was 30 years.
"It cannot be prolonged," he said.
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Exiled opposition group the National Council of Resistance of Iran (NCRI) voiced outrage that the case was being closed.
"The Iranian Resistance strongly protests and condemns the closure of the file on the assassination of Professor Kazem Rajavi," it said in a statement.
The NCRI, headed by Maryam Rajavi since her husband Massoud - Kazem's brother - disappeared in 2003, insisted that instead of closing the case, Switzerland should issue a range of international arrest warrants for Iran's top leadership.
It mentioned among others President Hassan Rouhani and Iran's supreme leader Ayatollah Ali Khamenei, whom it accused of having a role in commissioning and ordering the assassination.
"International arrest warrants must be issued for those who ordered and perpetrated this terrorist murder. Justice must be carried out," it said.
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