Swedish prosecutors said Wednesday they had released a woman suspected of holding her own son captive in their apartment for decades, but that the investigation was continuing.

The woman, who is 70, was arrested this week with police saying her now 41-year-old son had been "locked up for a very long time" in an apartment in a southern Stockholm suburb.

According to reports quoting an unnamed relative, he had been kept in the apartment for 28 years after being  taken out of school.

The relative, later identified as the man's sister, had found him on Sunday after their mother had been taken to hospital, daily Expressen reported.

The man, who was immediately hospitalised for surgery, had infected sores on his legs and could barely walk, media said. He had almost no teeth, limiting his ability to speak.

After police were alerted the woman was arrested on suspicion of illegal deprivation of liberty and causing bodily harm. She had denied the allegations.

"After the man was questioned during the afternoon today and from what else has been uncovered during the investigation the case has weakened against the woman," prosecutor Emma Olsson said in a statement on Wednesday.

Judging that the woman would not be able to damage the investigation, coupled with the weakened suspicions, Olsson said she had been released.

However Olsen stressed "the investigation has not been closed and the suspicions of a crime remain".

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.

A woman in Sweden suspected of holding her son captive inside their apartment for decades has been arrested, police said on Tuesday, with reports claiming the man was found undernourished with infected sores on his body and almost no teeth.

Stockholm police spokesman Ola Osterling told AFP the man had been "locked up for a very long time" in the apartment in a southern Stockholm suburb but refused to comment on local reports he had been held for 28 years.

Neighbours said they had not seen the son, now 41, for years -- or ever. 

Tove Boman, a 24-year-old who lives in the building next door, told AFP she had only seen the mother. "I grew up here so I've always known who she is and recognised her. She's a little strange," she said.

The Expressen and Aftonbladet newspapers reported that the woman had taken her son out of school when he was 12 and kept him locked inside the apartment since then.

An unnamed relative found the man on Sunday after the mother had been taken to hospital, Expressen reported.

The man had infected sores on his legs, could barely walk, and had almost no teeth, limiting his ability to speak, the reports said.

Osterling would not comment on those details, saying only: "The man is in hospital. His injuries are not life-threatening."

The woman is suspected of illegal deprivation of liberty and causing bodily harm -- allegations she denies.

Urine, dirt and dust 

Media reports suggested the mother was overly protective after having lost her firstborn son when he was three years old. 

The relative said the mother had convinced the boy that everybody was out to get them and that she was the only one who could protect them.

The relative also told the Dagens Nyheter newspaper she had contacted social services several times over the years but was told no crime had been committed because the man was not physically locked up.

Police tape was stretched across the apartment's door on Tuesday, and officers were seen leaving the scene around midday, an AFP photographer said.

The relative said the apartment, in a non-descript grey and yellow low-rise building in the working class suburb of Handen, looked like it had not been cleaned in years.

"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 hallways.

"I'm in shock, brokenhearted, but at the same time relieved. I've been waiting for this day for 20 years."

Police are due to question the man and his mother in the coming days to determine what had happened.

How could no one have noticed?

Neighbours contacted by AFP expressed shock that the man could have been hidden for so long.

Kenth Svedberg said he had noticed an "unpleasant odour" coming from the apartment but it was "nothing I thought very much about".

"What's so scary is that it's gone on for so many, many years," he told AFP.

Many neighbours wondered why neither social services, the school nor any other authority ever checked in on the boy over the years.

One woman told Aftonbladet she found it odd that their window was never open, and the same candlestick had been in the window for 30 years.

"But what can you do? How do you know what's going on behind closed doors? It's all so awful, you can't believe it's true," she said.