Police believe a body found on Wednesday evening is that of a missing disabled 22-year-old man.
James Hughes went missing on Friday evening along with his mother, Heather Wardle, who was found dead in Redditch, Worcestershire, three days later.
Police have a further 12 hours to question a man and a youth arrested on suspicion of murder.
It is understood that the arrested man is 36-year-old Brian Kirby, the partner of Ms Wardle.
Ms Wardle, 39, was found with a ligature on her neck near her home in Church Hill.
The second body was found at the family's Redditch home.
On Thursday evening officers were granted 12 extra hours to question the man and youth.
Police have cordoned off part of Loxley Close in Church Hill, where Ms Wardle and her son lived.
'Sad case'
A white tent has also been erected by police in the back garden of the house.
The two arrested men are being questioned at police stations in Kidderminster and Redditch, West Mercia Constabulary said.
Police said it was expected to take some time to check dental records in order to formally identify the body found on Wednesday. A post-mortem examination is being carried out.
HUNT FOR MOTHER AND SON
Both Mr Hughes and his mother were reported missing on Saturday, but detectives have been trying to establish when the last sighting of Mr Hughes, who had a mental age of 18 months and needed 24-hour care, was.
They believe his mother died within 24 hours of leaving her home on Friday evening, when she had said she was going to visit a friend.
Detectives are treating her death as "unexplained".
Supt Jane Horwood, of West Mercia Constabulary, said it was one of the saddest cases the force had ever dealt with.
"What started out as a missing persons inquiry has tragically become an investigation into two deaths," she said.
"While we never gave up hope of finding both Heather and James alive, we were extremely concerned about both of them right from the start, and our fears for James obviously increased after the discovery of his mother's body.
"This is without doubt one of the saddest cases West Mercia Constabulary has dealt with and the events of the last few days have left residents of Church Hill and the wider Redditch community shocked."
Ms Wardle was also mother to three other children: Daniel, 17, Callum, eight and six-year-old Luke.
A spokesperson for Worcestershire County Council, which runs the social care services in the area, said the authority was "deeply saddened and shocked" by what had happened.
She said: "We are co-operating fully with all of the agencies involved and await the outcome of investigations being carried out by police and the results of the post-mortem."
She added that the council was unable to comment on individual cases.
The local church has been open for people to pray for Ms Wardle and Mr Hughes.
Deacon Tony Martin, of St Andrew's Methodist Church, said: "It's a community that's grieving.
"I think they have had to come to terms with a lot of things happening very quickly."
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