Isaak has been imprisoned in Eritrea since 2001.
Press freedom group Reporters Without Borders (RSF) says he and his colleagues detained at the same time are now the journalists imprisoned the longest in the world.
In October last year, Sweden's RSF chapter – which considers Isaak a prisoner of conscience who has been held without charge – filed a complaint with Swedish authorities.
It was directed at Eritrea's President Isaias Afwerki and seven other high-ranking political leaders including Foreign Minister Osman Saleh Mohammed, accusing them of “crimes against humanity, enforced disappearance, torture and kidnapping”.
“Based on what has been presented I have reason to believe that crimes have been committed and are still being committed,” public prosecutor Karolina Wieslander told AFP.
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The crimes would fall under a 2014 law enabling prosecutions for human rights crimes committed abroad, but Wieslander said she had decided nonetheless not to open an investigation.
“In my assessment there are in practice no prospects that such an investigation… would get to the point where I could bring charges against one or more people,” Wieslander said.
The main reason, she said, was that even though some evidence could potentially be gathered in Sweden and other countries, investigation measures would ultimately also need to be taken in Eritrea.
That would require permission and cooperation from Eritrean authorities, and since the accused are part of the political leadership in Eritrea, the chances of that happening are minimal, according to Wieslander.
The press rights group slammed the decision as “outrageous” in a social media post and said the reasoning was “odd”.
“Are you supposed to refrain from investigating crimes where the suspects don't cooperate?” Björn Tunbäck, coordinator for RSF Sweden's work on the Isaak case, said in a statement.
On September 23rd, 2001, Isaak was arrested shortly after the Eritrean newspaper he founded, Setit, published articles demanding political reforms.
Isaak had fled to Sweden in 1987 during Eritrea's struggle against Ethiopia which eventually led to independence in 1993. He returned in 2001 to help shape the media landscape.
RSF ranks Eritrea as the world's third most repressive country when it comes to press freedom, behind North Korea and Turkmenistan.