The mother, who denied the allegations, was initially suspected of illegal deprivation of liberty and causing bodily harm.
“During the investigation, we did not find any evidence that proves that there has been any crime,” prosecutor Emma Olsson told Aftonbladet on Thursday.
According to reports quoting the man's sister, the son, a 41-year-old man, had been kept in the apartment in a Stockholm suburb for 28 years after being taken out of school.
The sister had found him on Sunday after their mother had been taken to hospital, Expressen reported. It was reported that the man, who was immediately hospitalised for surgery, had infected sores on his legs and could barely walk. He had almost no teeth, limiting his ability to speak.
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The case immediately garnered widespread attention in Sweden, as the sister painted a bleak picture of the state of the man and the apartment
“There was urine, dirt and dust. It smelled rotten,” she told Expressen, adding that she had to wade through piles of rubbish to get through the hallway.
Neighbours contacted by AFP also expressed shock that the man could have been hidden for so long.
The son's counsel did not want to answer any questions from the media on Wednesday. The TT newswire said it had also sought the mother's lawyer for a comment.
The investigation continues.