Swedish police have detained a woman suspected of holding her son captive in their flat for "a long time," a spokesman said on Tuesday.
The man, said to be in his early 40s, was found on Sunday in a flat in Haninge, a municipality south of the capital, after a female relative alerted police.
The man is currently in hospital receiving treatment for "injuries that were not life-threatening," Stockholm region police spokesman Ola Osterling said.
Stockholm tabloids Aftonbladet and Expressen that broke the story late Monday reported that the man could have been held illegally in the flat for almost 30 years.
The man was reportedly malnourished, had lost his teeth and had large sores on his body.
The female relative who alerted police told Expressen that the man's mother had "stolen his life."
She said the man was living in squalor. She entered the flat on Sunday after learning that his mother was not at home.
The Swedish Prosecution Authority said the mother, who was in her 70s, was being investigated on suspicion of illegal deprivation of liberty, causing gross bodily harm and causing illness.
Prosecutor Emma Olsson said the mother had denied the allegations.
"We will map the man's life and circumstances, and conduct a number of interviews," Olsson said in a statement.
The prosecutor must latest by midday Thursday request a court decision to place the mother in pre-trial detention.
Sources quoted by Aftonbladet said that the mother was "extremely over-protective" and took her son out of school when he was around age 12.